Hot Flare Onsets

Nugget
Number:	382
1st Author:	Hugh Hudson
2nd Author:	et al.
Published:	27 July 2020
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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, gyrosynchrotron radiation, and the acceleration of the coronal mass ejection if one occurs. 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

Figure 1: Three panels showing (a) the two GOES fluxes, soft and hard; (b) the background-subtracted emission measure and temperature, and (c) the correlation plot show the joint variation of thee two parameters. The hot onset shows up clearly as the blue points, clearly preceding the impulsive phase as marked on the left panel by a dotted vertical line at hard X-ray onset.

The time-series points in the two right panels of the figure have the same color-coded time range; the main flare development proceeds from green to red, including the impulsive phase

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