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The Hale Sector Boundary links flares to global magnetic fields

Hhudson — Fri, 09 Oct 2026 19:53:34 GMT

Conclusions

← Older revision		Revision as of 19:53, 9 October 2026
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	[https://en.wikipedia.org/wiki/Heliospheric_current_sheet heliospheric current sheet].		[https://en.wikipedia.org/wiki/Heliospheric_current_sheet heliospheric current sheet].

	[[File:537f1.png\|center\|thumb\|~~800px~~\|caption\|Figure 1: <i>		[[File:537f1.png\|center\|thumb\|600px\|caption\|Figure 1: <i>
	Figure 1. The extremes of solar magnetism. Left, the "salt-and-pepper" of		Figure 1. The extremes of solar magnetism. Left, the "salt-and-pepper" of
	the polarity distribution in the photosphere.		the polarity distribution in the photosphere.
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	active regions.		active regions.

	[[File:537f2.png\|center\|thumb\|~~800px~~\|caption\|Figure 2: <i>		[[File:537f2..png\|center\|thumb\|600px\|caption\|Figure 2: <i>
	Two examples from the		Two examples from the
	[https://www.astro.gla.ac.uk/users/iain/hsb/ HSB Web tool].		[https://www.astro.gla.ac.uk/users/iain/hsb/ HSB Web tool].
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	where opposing polarities prevail and may enable		where opposing polarities prevail and may enable
	[https://svs.gsfc.nasa.gov/20310 magnetic reconnection].		[https://svs.gsfc.nasa.gov/20310 magnetic reconnection].

			This Nugget describes our [interactive HSB database], but if you are
			interested there have been earlier Nuggets: [1], [2]

	== Historical note ==		== Historical note ==
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	== References ==		== References ==

	[1] [https://ui.adsabs.harvard.edu/abs/2011ApJ...733...49S/ "Flaring Hale Sector		[1] [https://ui.adsabs.harvard.edu/abs/2011ApJ...733...49S/ "Flaring Hale Sector Boundaries"]
	Boundaries"]

	[2] [https://ui.adsabs.harvard.edu/abs/2018A&A...618A...9L/ "The association of the Hale sector boundary with RHESSI solar flares and active longitudes"]		[2] [https://ui.adsabs.harvard.edu/abs/2018A&A...618A...9L/ "The association of the Hale sector boundary with RHESSI solar flares and active longitudes"]

The Hale Sector Boundary links flares to global magnetic fields

Hhudson — Fri, 09 Oct 2026 19:39:02 GMT

No. 537 initial upload

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{{Infobox Nugget
|name = Nugget
|title = The Hale Sector Boundary links flares to global magnetic fields
|number = 537
|first_author = Iain HANNAH
|second_author = Hugh HUDSON
|publish_date = October 12, 2026
|next_nugget =
|previous_nugget = {{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::536]]}}
}}

== Introduction ==

When spacecraft first began to observe the interplanetary field, they found
a remarkable property: the radial component appeared to alternate polarities on a few-day time scale.
This became known as the "sector structure", and can be understood from
the comparisons in Figure 1 (right panel).
Often there are two or four sectors per solar rotation, and
the scale of the homogenous field structures is many solar radii.
At the other extreme, the magnetic field at the surface of the Sun (left
panel) shows abundant fine structure "salt and pepper", at scales
"measureless to man" - including the fine structures of solar flares.
The surface separating the two polarities lies within the
[https://en.wikipedia.org/wiki/Heliospheric_current_sheet heliospheric current sheet].

[[File:537f1.png|center|thumb|800px|caption|Figure 1: 
Figure 1. The extremes of solar magnetism. Left, the "salt-and-pepper" of
the polarity distribution in the photosphere.
Note how the systematic EW patterns differ between the NS hemispheres.
Right, the iconic view of the heliospheric current sheet, which forms a
warped surface separating the two (+/-) polarities.
Because of the warps produced by field asymmetries, a fixed obsesrver see
alternating sectors.
]]

In this Nugget we describe the "Hale sector boundary" (Ref. [1])
and provide an interactive tool showing how it links these two
extreme spatial scales.
We define the Hale boundary as the "portions
of the heliospheric magnetic-field polarity inversion structure
where the polarity change has the same sense as the leading-to-following
polarity transition of sunspot groups in that hemisphere" (Ref. [2])
This refers to the amazing hemispheric preference for the "leading" sunspot,
in the sense of first appearance as the Sun rotate, to have the opposite
polarity from the "trailing" sunspots in a group.
This is guaranteed to be somewhat confusing, but Figure 2 shows
examples that make it all very clear.
These are from our Web-based
[https://www.astro.gla.ac.uk/users/iain/hsb/ interactive database]
of Hale sector boundaries, solar flares, and
active regions.

[[File:537f2.png|center|thumb|800px|caption|Figure 2: 
Two examples from the
[https://www.astro.gla.ac.uk/users/iain/hsb/ HSB Web tool].
The purple symbols show where the Hale sector boundary occurs as the
heliospheric current sheet sweeps below the solar equator; green dots
show active regions, and red letters show flare locations.
]]

== Conclusions ==

The tendency for the smallest magnetic features to occur at where the
largest-scale polarity structures meet is remarkable, but with a sketch
one can see that the Hale boundary, and not the non-Hale boundary, is
where opposing polarities prevail and may enable
[https://svs.gsfc.nasa.gov/20310 magnetic reconnection].

== Historical note ==

The authors became interested in this subject via the encouragement of our late
friend [https://solarnews.aas.org/2025/obituary-leif-svalgaard-1942-2025/ Leif Svalgaard].

== References ==

[1] [https://ui.adsabs.harvard.edu/abs/2011ApJ...733...49S/ "Flaring Hale Sector
Boundaries"]

[2] [https://ui.adsabs.harvard.edu/abs/2018A&A...618A...9L/ "The association of the Hale sector boundary with RHESSI solar flares and active longitudes"]

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Hhudson — Fri, 09 Oct 2026 19:23:30 GMT

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Hhudson — Fri, 09 Oct 2026 19:22:50 GMT

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Photospheric flows in Sun-as-a-star EUV Doppler spectra

Hhudson — Fri, 02 Oct 2026 20:16:15 GMT

EVE power spectra

← Older revision		Revision as of 20:16, 2 October 2026
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	[[File:536f1.png\|center\|thumb\|450px\|caption\|Figure 1: <i>		[[File:536f1.png\|center\|thumb\|450px\|caption\|Figure 1: <i>
	Time series of EVE 10-s Lyman-beta spectra, showing intensity and ~~Dopper spectra~~		Time series of EVE 10-s Lyman-beta spectra, showing intensity and Doppler redshifts
	of residuals against the median for each 3-hour sequence.		of residuals against the median for each 3-hour sequence.
	The Doppler timeseries (lower) clearly shows the SDO satellite's orbital motion as		The Doppler timeseries (lower) clearly shows the SDO satellite's orbital motion as
	a diurnal effect		a diurnal effect
	([https://en.wikipedia.org/wiki/Geosynchronous_orbit geosynchronous orbit])		([https://en.wikipedia.org/wiki/Geosynchronous_orbit geosynchronous orbit])
	with about 3 km/s amplitude.		with about 3 km/s amplitude.
			One can also faintly see the effects of the first X-class event of Cycle 24, SOL2011-02-15.
	</i>]]		</i>]]

SolarNuggets

Hhudson — Tue, 29 Sep 2026 20:21:41 GMT

← Older revision		Revision as of 20:21, 29 September 2026
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	Welcome to the [[SolarNuggets]] collection, which extends the series of [[RHESSI]] Nuggets. The following is a time-ordered list of the latest Nuggets added to the HelioWiki. An [[:Category:Nugget\|alphabetical list of the SolarNuggets]] is also available as well as [[:Category:RHESSI Nugget List\|yearly lists]]. One can search on author, topic, IAU flare identifier, etc.). We welcome volunteer authors - please see our page of [[Help:For_Authors\| help for authors]] or just send an email to the Curator at (hugh.hudson@glasgow.ac.uk).		Welcome to the [[SolarNuggets]] collection, which extends the series of [[RHESSI]] Nuggets. The following is a time-ordered list of the latest Nuggets added to the HelioWiki. An [[:Category:Nugget\|alphabetical list of the SolarNuggets]] is also available as well as [[:Category:RHESSI Nugget List\|yearly lists]]. One can search on author, topic, IAU flare identifier, etc.). We welcome volunteer authors - please see our page of [[Help:For_Authors\| help for authors]] or just send an email to the Curator at (hugh.hudson@glasgow.ac.uk).

			{{Nugget Badge
			\|title = Photospheric flows in Sun-as-a-star EUV Doppler spectra
			\|number = 536
			\|first_author = Hugh HUDSON,
			\|second_author = Anne-Marie BROOMHALL and Lyndsay FLETCHER
			\|publish_date = 28 September 2026
			\|description = Photospheric convection dominates the mHz Doppler spectrum in the transition region
			\|image=Icon536.png}}

	{{Nugget Badge		{{Nugget Badge
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	\|first_author = Atul MOHAN		\|first_author = Atul MOHAN
	\|publish_date = 14 September 2026		\|publish_date = 14 September 2026
	\|description = Evidence ~~grow~~ for the association of ~~SDGREs~~ with CME/CME interactions		\|description = Evidence grows for the association of SGREs with CME/CME interactions
	\|image=Icon535.png}}		\|image=Icon535.png}}

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Hhudson — Mon, 28 Sep 2026 13:50:31 GMT

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Photospheric flows in Sun-as-a-star EUV Doppler spectra

Hhudson — Mon, 28 Sep 2026 07:34:59 GMT

EVE power spectra

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	\|number = 536		\|number = 536
	\|first_author = Hugh HUDSON		\|first_author = Hugh HUDSON
	\|second_author = Lyndsay FLETCHER		\|second_author = Anne-Marie BROOMHALL and Lyndsay FLETCHER
	\|publish_date = September 28, 2026		\|publish_date = September 28, 2026
	\|next_nugget =		\|next_nugget =
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	of 2011.		of 2011.
	In each 10-s sample, one can get a robust Doppler measurement, and over		In each 10-s sample, one can get a robust Doppler measurement, and over
	a 3-h span this corresponds to the frequency range 0.1-50 ~~mHJz~~.		a 3-h span this corresponds to the frequency range 0.1-50 mHz.
	This frequency range includes the		This frequency range includes the
	[http://soi.stanford.edu/press/ssu8-97/pmodes.html p-modes],		[http://soi.stanford.edu/press/ssu8-97/pmodes.html p-modes],
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	[https://en.wikipedia.org/wiki/Limb_darkening limb brightening].		[https://en.wikipedia.org/wiki/Limb_darkening limb brightening].

	[[File:536f1.png\|center\|thumb\|~~600px~~\|caption\|Figure 1: <i>		[[File:536f1.png\|center\|thumb\|450px\|caption\|Figure 1: <i>
	Time series of EVE 10-s Lyman-beta spectra, showing intensity and Dopper.		Time series of EVE 10-s Lyman-beta spectra, showing intensity and Dopper spectra
	The Doppler ~~series~~ clearly shows the SDO satellite's orbital motion as		of residuals against the median for each 3-hour sequence.
			The Doppler timeseries (lower) clearly shows the SDO satellite's orbital motion as
	a diurnal effect		a diurnal effect
	([https://en.wikipedia.org/wiki/Geosynchronous_orbit geosynchronous orbit])		([https://en.wikipedia.org/wiki/Geosynchronous_orbit geosynchronous orbit])
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	</i>]]		</i>]]

	Each 3-hour data series, one per day, then ~~produces~~ a power spectrum		Each 3-hour data series, one per day, then yields a power spectrum via
	via
	[https://en.wikipedia.org/wiki/Joseph_Fourier Fourier]		[https://en.wikipedia.org/wiki/Joseph_Fourier Fourier]
	[https://en.wikipedia.org/wiki/Fourier_transform transforms],		[https://en.wikipedia.org/wiki/Fourier_transform transforms] of residuals against median values,
	and the mean of these spectra (an "incoherent sum", since phase information		and the mean of these spectra (an "incoherent sum", since phase information
	is not retained) results in a very clean spectrum, based on typically 250		is not retained) results in a very clean spectrum, based on typically 250
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	ionization states.		ionization states.

	[[File:~~536f1~~.png\|center\|thumb\|600px\|caption\|Figure 1: <i>		[[File:536f2.png\|center\|thumb\|600px\|caption\|Figure 2: <i>
	Power spectra of O and Mg ~~lines~~, showing different ionization states.		Power spectra of Doppler motions in emission lines of O and Mg, showing different ionization states.
	The higher-temperature states tend systematically to		The higher-temperature states tend systematically to
	show less of the photospheric Harvey spectrum.		show less of the photospheric Harvey spectrum.
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	The amplitude of the flows is consistent with the "microturbulent"		The amplitude of the flows is consistent with the "microturbulent"
	excess line widths, but the EVE data show that the Doppler motions are		excess line widths, but the EVE data show that the Doppler motions are
	not turbulent in ~~nature~~.		not turbulent in the sense of a Kolmogorov model.


	== References ==		== References ==

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Hhudson — Sun, 27 Sep 2026 10:00:40 GMT

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Hhudson — Sun, 27 Sep 2026 10:00:22 GMT

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Photospheric flows in Sun-as-a-star EUV Doppler spectra

Hhudson — Sun, 27 Sep 2026 09:59:50 GMT

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	Figure 2 shows the 2011 spectra for oxygen and magnesium ions at different		Figure 2 shows the 2011 spectra for oxygen and magnesium ions at different
	ionization states.		ionization states.

			[[File:536f1.png\|center\|thumb\|600px\|caption\|Figure 1: <i>
			Power spectra of O and Mg lines, showing different ionization states.
			The higher-temperature states tend systematically to
			show less of the photospheric Harvey spectrum.
			</i>]]

	== The photospheric motions and the Harvey function ==		== The photospheric motions and the Harvey function ==

Photospheric flows in Sun-as-a-star EUV Doppler spectra

Hhudson — Sun, 27 Sep 2026 09:56:41 GMT

No. 536 without figures

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{{Infobox Nugget |name = Nugget
|title = Photospheric flows in Sun-as-a-star EUV Doppler spectra
|number = 536
|first_author = Hugh HUDSON
|second_author = Lyndsay FLETCHER
|publish_date = September 28, 2026
|next_nugget =
|previous_nugget = {{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::535]]}}
}}

== Introduction ==

This Nugget describes a new application of the
[https://lasp.colorado.edu/eve/ SDO/EVE]
Sun-as-a-star EUV spectroscopy.
These very precise data allow us to get a first look at the mean
Doppler variability revealed by many emission lines covering chromospheric
to coronal temperature ranges.
A first study of this kind (Ref. [1]) averages Doppler measurements for
the entire year 2011, and finds characteristic and unexpected patterns.
These features relate to mean properties, blithely ignoring all of
the beautiful structure and dynamics of solar EUV images.
Note that the EUV spectra do not show the photosphere directly, with
dominant contributions from the
[https://solarscience.msfc.nasa.gov/t_region.shtml transition region]
and low corona.

== What do power spectra show us? ==

EVE operational patterns give us 3-hour contiguous data runs for most days
of 2011.
In each 10-s sample, one can get a robust Doppler measurement, and over
a 3-h span this corresponds to the frequency range 0.1-50 mHJz.
This frequency range includes the
[http://soi.stanford.edu/press/ssu8-97/pmodes.html p-modes],
standing waves in the solar interior; the
[http://hmi.stanford.edu/hminuggets/?p=3287 chromospheric 3-minute oscillations],
a feature of solar sunspot activity, and photospheric flows due to
convection, prominently the
[https://en.wikipedia.org/wiki/Solar_granule granulation] in the
[https://scied.ucar.edu/learning-zone/sun-space-weather/surface-of-the-sun photosphere].

At the highest frequencies (above 10 mHz), we do not have much stellar
Doppler observation yet, basically because the stars are faint and require
longer intgration times for such measurements.
Accordingly the EVE Sun-as-a-star spectra are pioneering.
What we can see in stellar observations is
[https://en.wikipedia.org/wiki/Microturbulence "microturbulence"], a catchall
phrase that describes the excess widths of spectral lines.
This is an integral property of the power spectrum, present even in
non-convective stars, and the significance of the EVE spectra is that one can
get a detailed spectrum of the motions.
This in principle can help to understand their physics.

== EVE power spectra ==

To analyze the EVE spectra, we first find the line centroid wavelength in each
10-s integration.
This is done with a six-parameter function consisting of a Gaussian plus
a quadratic background continuum.
In practice this effectively eliminates the effects of line blends in
the EVE spectra, which do not have high spectral resolution.
Figure 1 shows a month of EVE data in the
[https://en.wikipedia.org/wiki/Lyman_series Lyman-beta] line, one of the
EVE lines with chromospheric properties.
Note however that it is optically thick, and that its contributions to
Sun-as-a-star signals may actually show a range of heights, especially
towards the limb.
Because of projection effects, horizontal motions would have very strong
[https://en.wikipedia.org/wiki/Limb_darkening limb brightening].

[[File:536f1.png|center|thumb|600px|caption|Figure 1: 
Time series of EVE 10-s Lyman-beta spectra, showing intensity and Dopper.
The Doppler series clearly shows the SDO satellite's orbital motion as
a diurnal effect
([https://en.wikipedia.org/wiki/Geosynchronous_orbit geosynchronous orbit])
with about 3 km/s amplitude.
]]

Each 3-hour data series, one per day, then produces a power spectrum
via
[https://en.wikipedia.org/wiki/Joseph_Fourier Fourier]
[https://en.wikipedia.org/wiki/Fourier_transform transforms],
and the mean of these spectra (an "incoherent sum", since phase information
is not retained) results in a very clean spectrum, based on typically 250
good daily spectra from a single year.
Figure 2 shows the 2011 spectra for oxygen and magnesium ions at different
ionization states.

== The photospheric motions and the Harvey function ==

All of the lines in Figure 2, and indeed the 20 other lines in the sample
analyzed in Ref. [1] show similar features: they are broad continua,
monotonically decreasing with frequency, and they show the characteristic
Harvey profile (Ref. [2]) that nicely describes the continuum of
photospheric motions.
The Harvey spectrum is a Lorentzian, closely related to shot noise: white at
low frequencies, and rolling over at a characteristic frequency (the inverse
of the mean time scale of the source motions.
The basic Harvey spectrum needs a generalization, since the EVE data
clearly do not show a f-2 tail at high frequencies, but a
much steeper power law that is quite well defined.
In no case do the EVE spectra show broad spectral peaks either
in the photospheric p-mode 5-minute range, or the chromospheric 3-minute range.
The spectra also show no trace of a Kolmogorov f-5/3 turbulence.

== Conclusions ==

The EVE spectra look entirely photospheric and probably result from
basically horizontal motions, which couple to MHD modes in the solar
atmosphere.
Notably the Harvey spectrum near 8 mHz, which must reflect the granulation,
becomes less important for the lines with coronal opacity.
The amplitude of the flows is consistent with the "microturbulent"
excess line widths, but the EVE data show that the Doppler motions are
not turbulent in nature.

== References ==

[1] [https://ui.adsabs.harvard.edu/abs/2026arXiv260705108H "Signatures of photospheric convection throughout the solar atmosphere"]

[2] [https://ui.adsabs.harvard.edu/abs/1993ASPC...42..111H "Chromospheric Oscillations and the Background Spectrum"]

SolarNuggets

Hhudson — Thu, 17 Sep 2026 20:39:05 GMT

Added No. 535

← Older revision		Revision as of 20:39, 17 September 2026
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	Welcome to the [[SolarNuggets]] collection, which extends the series of [[RHESSI]] Nuggets. The following is a time-ordered list of the latest Nuggets added to the HelioWiki. An [[:Category:Nugget\|alphabetical list of the SolarNuggets]] is also available as well as [[:Category:RHESSI Nugget List\|yearly lists]]. One can search on author, topic, IAU flare identifier, etc.). We welcome volunteer authors - please see our page of [[Help:For_Authors\| help for authors]] or just send an email to the Curator at (hugh.hudson@glasgow.ac.uk).		Welcome to the [[SolarNuggets]] collection, which extends the series of [[RHESSI]] Nuggets. The following is a time-ordered list of the latest Nuggets added to the HelioWiki. An [[:Category:Nugget\|alphabetical list of the SolarNuggets]] is also available as well as [[:Category:RHESSI Nugget List\|yearly lists]]. One can search on author, topic, IAU flare identifier, etc.). We welcome volunteer authors - please see our page of [[Help:For_Authors\| help for authors]] or just send an email to the Curator at (hugh.hudson@glasgow.ac.uk).

			{{Nugget Badge
			\|title = Why do only some fast CMEs produce sustained gamma-ray emissions?
			\|number = 535
			\|first_author = Atul MOHAN
			\|publish_date = 14 September 2026
			\|description = Evidence grow for the association of SDGREs with CME/CME interactions
			\|image=Icon535.png}}

	{{Nugget Badge		{{Nugget Badge

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Hhudson — Thu, 17 Sep 2026 20:35:23 GMT

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Why do only some fast CMEs produce sustained gamma-ray emissions?

Hhudson — Thu, 17 Sep 2026 20:33:10 GMT

author's fixes

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	\|title = Why do only some fast CMEs produce sustained gamma-ray emissions?		\|title = Why do only some fast CMEs produce sustained gamma-ray emissions?
	\|number = 535		\|number = 535
	\|first_author = ~~Ratul~~ MOHAN		\|first_author = Atul MOHAN
	\|publish_date = September 14, 2026		\|publish_date = September 14, 2026
	\|next_nugget =		\|next_nugget =
	\|previous_nugget = {{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::~~534~~]]}}		\|previous_nugget = {{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::533]]}}
	}}		}}

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	[https://www.spaceweather.gov/phenomena/coronal-mass-ejections		[https://www.spaceweather.gov/phenomena/coronal-mass-ejections
	CMEs] and their shocks are likely accelerators of the high-energy protons		CMEs] and their shocks are likely accelerators of the high-energy protons
	that are transported sunward to generate SGREs.		that are transported sunward to generate SGREs
	~~last~~ over 3h (>3h-SGREs) after the impulsive phase.		lasting over 3h (>3h-SGREs) after the impulsive phase.
	Nevertheless, many fast CMEs ~~are~~ do not cause SGREs.		Nevertheless, many fast CMEs do not cause SGREs.
	Some ARs can produce several SGRE-associated CMEs during		Some ARs can produce several SGRE-associated CMEs during
	their lifetime within a gap of a few days.		their lifetime within a gap of a few days.
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	== Definitions and the major AR catalog ==		== Definitions and the major AR catalog ==


	We studied CMEs associated with flares stronger than GOES C-class		We studied CMEs associated with flares stronger than GOES C-class
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	The following definitions were adopted to		The following definitions were adopted to
	identify the relevant active regions and periods of clustered		identify the relevant active regions and periods of clustered
	activity in our [https://cdaw.gsfc.nasa.gov/pub/atul/majorARs/ catalog]:		CME-activity in our [https://cdaw.gsfc.nasa.gov/pub/atul/majorARs/ catalog]:

	1. Major AR: An AR that produced >1 CME-associated-flare of class M or stronger.		1. Major AR: An AR that produced >1 CME-associated-flare of class M or stronger.
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	[[File:535f3.png\|center\|thumb\|800px\|caption\|Figure 1: <i>		[[File:535f3.png\|center\|thumb\|800px\|caption\|Figure 1: <i>
	~~Figure 1.~~ CME occurrence in two major ARs, noting the speeds of the fast CMEs.		CME occurrence in two major ARs, noting the speeds of the fast CMEs.
			Pink boxes identify CME-clusters in the ARs.
	</i>]]		</i>]]

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	Our detailed case		Our detailed case
	studies on two very active major ARs lead us to the hypothesis that		studies on two very active major ARs lead us to the hypothesis that
	an extremely fast (>3000 km s-1) CME may produce an SGRE through the		an extremely fast (>3000 km/s) CME may produce an SGRE through the
	strength of its shock alone, whereas moderately fast ones (<2000		strength of its shock alone, whereas moderately fast ones (<2000
	km/s) require a CME/CME interaction below 10 R<sub>Sun</sub>, as evidenced by		km/s) require a CME/CME interaction below 10 R<sub>Sun</sub>, as evidenced by

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Why do only some fast CMEs produce sustained gamma-ray emissions?

Hhudson — Wed, 16 Sep 2026 16:31:33 GMT

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	major AR.		major AR.

	[[File:~~535f1~~.png\|center\|thumb\|800px\|caption\|Figure 1: <i>		[[File:535f3.png\|center\|thumb\|800px\|caption\|Figure 1: <i>
	Figure 1. CME occurrence in two major ARs, noting the speeds of the fast CMEs.		Figure 1. CME occurrence in two major ARs, noting the speeds of the fast CMEs.
	</i>]]		</i>]]

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Why do only some fast CMEs produce sustained gamma-ray emissions?

Hhudson — Wed, 16 Sep 2026 16:24:20 GMT

Created page with "{{Infobox Nugget |name = Nugget |title = Why do only some fast CMEs produce sustained gamma-ray emissions? |number = 535 |first_author = Ratul MOHAN |publish_date = September 14, 2026 |next_nugget = |previous_nugget = {{#ask: Category:Nugget RHESSI Nugget Index::534}} }} == Introduction == Certain strong solar flares may accelerate particles producing intense gamma-ray emissions during their impulsive phase. In some flares, however, gamma rays with energies >..."

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{{Infobox Nugget |name = Nugget
|title = Why do only some fast CMEs produce sustained gamma-ray emissions?
|number = 535
|first_author = Ratul MOHAN
|publish_date = September 14, 2026
|next_nugget =
|previous_nugget = {{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::534]]}}
}}

== Introduction ==

Certain strong solar flares may accelerate particles producing
intense gamma-ray emissions during their impulsive phase. In some
flares, however, gamma rays with energies >100 MeV continue to
arrive for several hours well beyond the impulsive phase. These
events are called sustained gamma-ray emission events (SGREs),
and require >300 MeV protons to interact in the dense photosphere.
Fast (>1000 km/s) coronal mass ejections
[https://www.spaceweather.gov/phenomena/coronal-mass-ejections
CMEs] and their shocks are likely accelerators of the high-energy protons
that are transported sunward to generate SGREs.
last over 3h (>3h-SGREs) after the impulsive phase.
Nevertheless, many fast CMEs are do not cause SGREs.
Some ARs can produce several SGRE-associated CMEs during
their lifetime within a gap of a few days.
Such periods of high CME-occurrence rates often cause CME-CME
interactions, resulting in particle acceleration
(Ref. [1])
These observations raise two central questions which our work (Ref. [2])
addresses:

1. What makes a major CME-productive AR SGRE-producing?
a. Is there a role for temporal clustering of CME events?
2. Why only some of the fast CMEs from these ARs associated with SGRE?
a. Is there a role for CME-CME interactions?

== Definitions and the major AR catalog ==

We studied CMEs associated with flares stronger than GOES C-class
during 2011-2019, covering the period in solar cycle 24, for which
[https://science.nasa.gov/mission/fermi/ Fermi] gamma-ray data is available.
The following definitions were adopted to
identify the relevant active regions and periods of clustered
activity in our [https://cdaw.gsfc.nasa.gov/pub/atul/majorARs/ catalog]:

1. Major AR: An AR that produced >1 CME-associated-flare of class M or stronger.
2. SGRE-producing AR: A major AR that produced at least one >3h-SGRE.
3. SGRE-lacking AR: A major AR that did not produce any >3h-SGRE.
4. CME cluster: A sequence of CMEs, each associated with a flare stronger than C class, from the same AR with a waiting time <1 day.
5. Fast CME epoch: A 2-day period centered around the fast CME onset time.

Figure 1 shows an example of CME event occurrence
plots and identified clusters in an SGRE-producing and an SGRE-lacking
major AR.

[[File:535f1.png|center|thumb|800px|caption|Figure 1: 
Figure 1. CME occurrence in two major ARs, noting the speeds of the fast CMEs.
]]

We identified 76 major ARs during the study period, of which 12
produced all of the 22 >3h-long SGREs reported in Cycle 24.
Overall, SGRE-associated CMEs generally tend
to occur in CME clusters during epochs with high chances of interaciton
with the preceeding CME.

== What makes some fast CMEs in clusters SGRE-associated? ==

A likely candidate here could be the CME-CME interaction, which is
known to often cause enhanced proton flux in >10MeV channels,
observed near Earth. We investigated this possibility in detail for
two especially major ARs that produced multiple long-duration SGREs.
See Ref. [2] for full detail, and refer to Figure 2 for an artist's
conception of the process.

[[File:535f2.png|center|thumb|800px|caption|Figure 2: 
Figure 2. CME-CME interaction. A fast CME (red)
interacts with a preceding CME (blue) accelerating protons >300
MeV, within a distance of 10R. Sunward-propagating protons (yellow)
produce SGREs at the location marked by bright flashes, while
outward-propagating protons (red) produce major SEP events. The
right panel summarises the conditions hypothesized to favor SGRE
production by fast CMEs.
]]

== Conclusions ==

Our detailed case
studies on two very active major ARs lead us to the hypothesis that
an extremely fast (>3000 km s-1) CME may produce an SGRE through the
strength of its shock alone, whereas moderately fast ones (<2000
km/s) require a CME/CME interaction below 10 RSun, as evidenced by
the CME height-time plots and coronagraph images for the selected events.
Testing the hypothesis against the Fermi LAT SGRE catalog gave a
high success rate, with just two outliers, in which cases other factors
of heliospheric preconditiong and local AR conditions may explain
the anomaly making them unique cases.

== References ==

[1] [https://ui.adsabs.harvard.edu/abs/2001ApJ...548L..91G/ "Radio Signatures of Coronal Mass Ejection Interaction: Coronal Mass Ejection Cannibalism?"]

[2] [https://ui.adsabs.harvard.edu/abs/2025SoPh..300..133M/abstract "Role of CME Clusters and CME-CME Interactions in Producing Sustained γ-Ray Emission Eents"]