User talk:Bdennis

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Introduction

Every Geostationary Operational Environmental Satellite (GOES) since 1976 has carried an X-Ray Sensor (XRS) consisting of a pair of ion chambers to measure the total solar flux in two wavelength bands, 0.5 - 4 and 1 - 8 Angstroms. These measurements recorded every 3 s (2 s since 2 December 2009) have been extremely valuable in providing a largely uninterrupted record of solar activity over the last 40 years. The absolute sensitivities of the instruments on the different spacecraft have been kept remarkably constant and the ABCMX flare classification scheme is widely used to this day. An IDL user-friendly GOES Workbench is available to plot the light curves for any time interval with or without background subtraction. The temperature and emission measure of the emitting flare plasma can also be readily estimated from the two-channel measurements assuming that the emitting plasma is all at the same temperature and has coronal abundances (Ref. [1]).

Initially, it was thought that the soft X-ray light curves as measured by GOES XRS were not particularly informative. The typical light curve rises relatively slowly during the impulsive phase seen in hard X-rays or microwaves. An example is shown in the top half of Figure 1. After reaching a peak near the end of the impulsive phase, it falls roughly exponentially with a somewhat longer time scale than on the rise. Neupert had noted that the soft X-ray rise was comparable to the time integral of the microwave flux, and Dennis and Zarro showed that a similar relation existed between soft and hard X-rays for some flares (Ref. [2]). This relationship is known as the Neupert Effect and can be best visualized by displaying the time derivative of the soft X-ray light curve as shown in the lower half of Figure 1. This time derivative has been used as a proxy for the hard X-ray or microwave light curves when they are not available.

The common explanation for the Neupert Effect is that the hard X-rays and microwaves are generated by electrons accelerated during the impulsive phase to 10s - 100s of keV and that these same electrons heat plasma to temperatures of ~10 to 50 MK that then radiates in soft X-rays. Since the heating can be very rapid on timescales of seconds or less while the plasma cooling time is likely to be much longer, the amount of hot plasma accumulates with time during the impulsive phase. Hence, at any given time during the flare, the amount of hot plasma and the soft X-ray flux emitted by it is proportional to the total energy of all the electrons that have been accelerated up to that time (Ref [3]).

We have already had several Nuggets on various aspects of this remarkable phenomenon - ([1. Harmonic Oscillations], [2. QPPs: Fermi/GBM Results], [3. Decimetric Pulsations], [4, HXR Pulsations], [5. Slow Magnetoacoustic Waves], [6, EVE/ESP and the Neupert Effect])

SOL2002-07-23

Figure 1: GOES soft X-ray light curves (top) and time derivative for the gamma-ray flare SOL2002-07-23.

Figure 2 (top): RHESSI light curves for seven energy bands between 3 keV and 1 MeV for the flare. (Bottom) Time derivative of the GOES soft X-ray light curves shown in Figure 1.

Figures 1 and 2 show the GOES light curves and their time derivatives for the X-class flare on 23 July 2002. Although the light curve, plotted here on a log scale, appears perfectly smooth, the time derivative reveals much detailed structure on the rise. When this event occurred, it was not clear if this fine structure was real or some type of artifact of the GOES data. It has now become certain, based on comparisons with simultaneous observations made with other instruments viewing similar wavelength ranges, that much of this fine structure is real and of solar origin. Prior to 2010, when GOES 13 was launched, the ability to detect this fine structure was severely compromised at soft X-ray fluxes exceeding ~10^-4 Watts m^-2 because of the automatic increase in the flux bin size at that level. This is evident in Figure 1, where the time derivative after 00:28 UT suffers from this coarse digitization at high flux levels. GOES 13 and GOES 15 have removed this problem and also improved the time cadence from 3 s to 2 s, thus allowing detailed structure in the time derivatives to be detected for even the most intense flares.

SOL2013-10-23

Figure 3: RHESSI 50 to 100 keV light curve and GOES and EVE/ESP time derivative light curves for the flare starting at 01:52 UT on 23 October 2013 (SOL2013-10-23)

An excellent example of the capabilities of the soft X-ray detector on the latest GOES spacecraft (GOES 15) is shown in Figure 3 for the flare that occurred on 28 October 2013. The RHESSI 50 - 100 keV hard X-ray light curve is shown on the same time axis as the time derivatives of the GOES and EVE/ESP light curves. Level 1 data were used from the EVE/ESP quad Si photodiode, which is sensitive in the 0.1 to 7 nm bandpass. This event has 5 or 6 sharp peaks in the hard X-ray light curves between 01:55 and 02:01 UT and the same number of peaks in the three time derivative curves showing a clear case of the Neupert Effect. However, more peaks are evident in the GOES time derivatives after this time and continue well into the decay of the event at least until 02:14 UT. Some of these are seen in the EVE/ESP time derivative curve as well but not all, presumably because the EVE/ESP quad diode is more sensitive to lower temperature plasma than GOES. But the question remains if all of the structure seen in the GOES time derivative curves is real or, as before, may be some artifact of GOES data even on the new spacecraft.

SOL2015-

Figure 3: GOES 13 and 15 light curves for flare on 11 March 2015 (SOL2015-03-15).

Figure 4: Time derivatives of GOES 13 and 15 light curves with cross-correlation plots for indicated time intervals.

The solar origin of structure in the GOES time derivative light curves during the decay phase of flares has been demonstrated since January 2015, when data from both GOES 13 and GOES 15 have become available for the same flares. Figures 4 and 5 show the time derivative of the light curves obtained simultaneously with GOES 13 and GOES 15. The soft X-ray detectors on these two spacecraft are nominally identical with the same flux digitization and readout systems. Consequently, the similar detailed structure seen in these two plots must be of solar origin. That there is similar structure in the two light curves can be seen from the cross-correlation plots for multiple time intervals during the decay phase of the event from the peak at 16:20 UT to as late as 17:00 UT. The clear multiple peaks in the cross-correlation plots show that the average time between peaks in the time derivative changes from ~16 s to 24 s to 20 s to ~35 s but apparently not in a systematic way.

Conclusions

1. Small features in GOES flare light curves are real and of solar origin.
2. The Neupert Effect occurs in many but not all events.
3. Peaks in the soft X-ray time derivative are not always correlated with hard X-ray peaks.
4. Soft X-ray fluctuations can continue well into the decay phase of the flare.
5. The time between peaks tends to be between 10 and 20 s but there is no sustained periodicity.

Nakariakov and Melnokov [Ref. 4] have divided the possible mechanisms for producing the observed fluctuations into the following general categories:

Load/unload Mechanisms

• Successive energy release in a given current sheet.
• Successive energy release in neighboring current sheets.

MHD Oscillations

• Triggering of energy release
• Modulation of energy release rate
• Acceleration and dynamics of electrons and ions
• Oscillations of magnetic loops.

References

[1] White, Thomas, and Schwartz (2005) "Updated Expressions for Determining Temperatures and Emission Measures from Goes Soft X-Ray Measurements"

[2] Dennis and Zarro (1993) "The Neupert effect - What can it tell us about the impulsive and gradual phases of solar flares?"

[3] Veronig et al. (2005) "Physics of the Neupert Effect: Estimates of the Effects of Source Energy, Mass Transport, and Geometry Using RHESSI and GOES Data"

[4] Nakariakov and Melnikov (2009) "Quasi-Periodic Pulsations in Solar Flares"