Hot Flare Onsets: Difference between revisions
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The basic soft X-ray time histories of solar flares often show them to have a "precursor" phase, in which the | The basic soft X-ray time histories of solar flares often show them to have a "precursor" phase, in which the | ||
[ | [https://www.ngdc.noaa.gov/stp/satellite/goes-r.html GOES/XRS] flux increases slightly, prior to the dominant | ||
impulsive phase in which we see hard X-rays, gyro synchrotron radiation, and the acceleration of an associated | impulsive phase in which we see hard X-rays, gyro synchrotron radiation, and the acceleration of an associated | ||
coronal mass ejection. | coronal mass ejection. | ||
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flux scale; these magnitude levels range from 10<sup>-8</sup> to 10<sup>-4</sup> W/m<sup>2</sup>, respectively, | flux scale; these magnitude levels range from 10<sup>-8</sup> to 10<sup>-4</sup> W/m<sup>2</sup>, respectively, | ||
Over this broad range, flare peak temperatures vary only weakly, typically 5-25 MK (10<sup>6</sup> Kelvin. | Over this broad range, flare peak temperatures vary only weakly, typically 5-25 MK (10<sup>6</sup> Kelvin. | ||
These temperatures are well above the temperature of the non-flaring [ | These temperatures are well above the temperature of the non-flaring [https://spaceplace.nasa.gov/sun-corona/en/ solar corona], | ||
resulting in the high contrast of flares in the solar X-ray time series captured by a GOES-type sensor. | resulting in the high contrast of flares in the solar X-ray time series captured by a GOES-type sensor. | ||
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This is true for weak events (B-class), for which the isothermal temperature never gets any higher, as well as | This is true for weak events (B-class), for which the isothermal temperature never gets any higher, as well as | ||
strong events (X-class), for which the peak temperature can be much higher. | strong events (X-class), for which the peak temperature can be much higher. | ||
Figure 1 shows a | Figure 1 shows a correlation plot of the time-series development for a particular flare, SOL. | ||
The two isothermal parameters (emission measure, closely related to the detected flux level, and temperature) | |||
exhibit a characteristic pattern essentially describing the | |||
[https://en.wikipedia.org/wiki/Neupert_effect Neupert effect] | |||
== Postscript == | == Postscript == | ||
Revision as of 16:30, 24 July 2020
| Nugget | |
|---|---|
| Number: | 382 |
| 1st Author: | Hugh Hudson |
| 2nd Author: | et al. |
| Published: | 27 July 2020 |
| Next Nugget: | TBD |
| Previous Nugget: | Extreme-Ultraviolet Late Phase of Solar Flares |
Introduction
The basic soft X-ray time histories of solar flares often show them to have a "precursor" phase, in which the GOES/XRS flux increases slightly, prior to the dominant impulsive phase in which we see hard X-rays, gyro synchrotron radiation, and the acceleration of an associated coronal mass ejection. Various signatures may appear: the gradual rise of a filament, prior to its eruption; spectroscopic hints of broad emission lines, weak flare-like events, and others. Because we cannot really predict flare occurrence yet, these phenomena have great potential for helping us to understand the direct cause of the event, if such a thing can be known.
The basic soft X-ray photometry of solar flares comes from NOAA"s GOES satellites, which began systematic flare observations from space in the 1970s and continue to the present with approximately the same instrumentation, detecting few-keV X-rays in two standard wavelength bands: 1-8 Â ("soft") and 0.5-4 Â ("hard"). With these two data one can make a crude but useful estimate of an isothermal temperature for the emitting flare plasma. As is well known, flares occur in a broad distribution of magnitudes, which NOAA simplifies by the "ABCMX" logarithmic flux scale; these magnitude levels range from 10-8 to 10-4 W/m2, respectively, Over this broad range, flare peak temperatures vary only weakly, typically 5-25 MK (106 Kelvin. These temperatures are well above the temperature of the non-flaring solar corona, resulting in the high contrast of flares in the solar X-ray time series captured by a GOES-type sensor.
A surprising finding
The simple GOES photometry led us (Ref.[1]) to a bit of a surprise: The flare onset, i.e. its first detectable appearance, systematically has a highly elevated isothermal temperature, typically in the range 10-15 MK. This is true for weak events (B-class), for which the isothermal temperature never gets any higher, as well as strong events (X-class), for which the peak temperature can be much higher. Figure 1 shows a correlation plot of the time-series development for a particular flare, SOL. The two isothermal parameters (emission measure, closely related to the detected flux level, and temperature) exhibit a characteristic pattern essentially describing the Neupert effect
Postscript
and only recently has instrumentation improved to the point at which still lesser decades of peak flux might require additional letters, such as Q and S for the 10-10 to 10-9 W/m2 range (Ref. [1]). This is
Acknowledgements
[1] "Flare Characteristics from X-ray Light Curves" Gryciuk