Hot Flare Onsets
From RHESSI Wiki
|1st Author:||Hugh Hudson|
|2nd Author:||et al.|
|Published:||27 July 2020|
|Previous Nugget:||Extreme-Ultraviolet Late Phase of Solar Flares|
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 wide range, the flare peak temperatures vary only weakly, typically over 5-25 MK (106 Kelvin from A- to X-class. 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.) 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 is considerably higher. Figure 1 illustrates the behavior of isothermal GOES model fits. It shows a correlation plot (c) of the time-series development for a particular flare, SOL2014-01-07 (M7). The two isothermal parameters (emission measure, closely related to the detected flux level, and temperature) exhibit a characteristic pattern essentially describing the Neupert effect
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. The hot onset is in blue and has a characteristic tendency to show no detectable temperature variation. Note that this is a low flux level and careful background subtraction is essential. Ref.  describes this phenomenon for a small range of different flares, including confirmation of the temperatures via RHESSI imaging spectroscopy. The article also uses AIA EUV imagery to locate the hot onset emission to the lower solar atmosphere, much of it in "footpoint" regions that subsequently develop into the flare proper.
An important implication
This onset interval, in the flare precursor, does not seem to exhibit "heating" in the sense of a temperature increase starting from either coronal (1-2 MK) or chromospheric (0.01 MK) material. Since this onset phase occurs by definition before any complicated flare structures have evolved, it should be a clear target for modelers to tackle with their well-developed capability for 1D radiation hydrodynamics simulations, such as RADYN (Ref. ).
The detection of hot onsets is at the limit of most current solar X-ray detectors (such as GOES) in spite of the relatively high X-ray flux levels. This suggests that more sensitive observations can make real contributions to our knowledge of flare dynamics. Only recently has instrumentation improved to the point at which the fluxes well below the ABCMX range can be usefully detected. Ref.  suggests adding Q and S for the next two decades below A, making a sequence QSABCMX that can reach down to the quiet Sun background level in the GOES energy range, the decades covering 10-10 to 10-9 W/m2 range.
The co-authors of Ref.  are Paulo J. A. Simões, Lyndsay Fletcher, Laura A. Hayes, and Iain G. Hannah.
 [https://ui.adsabs.harvard.edu/abs/1997ApJ...481..500C/abstract "Non-LTE Radiating Acoustic Shocks and CA II K2V Bright Points"}