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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 GOES 13 was launched in 2010) 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 use-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 (1968, ApJ, 153, 59) had noted that the soft X-ray rise was comparable to the time integral of the microwave flux and Dennis and Zarro (1992, Sol. Phys. 146, 177) showed that a similar relation existed with hard X-rays for some flares. This relationship 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 has been used as a proxy for the hard X-ray or microwave light curves when they are not available.

This similarity between the soft X-ray light curve and the time integral of both the hard X-ray and microwave light curves has been called the Neupert Effect. The common explanation 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 ([1], [2], [3], [4], [5]) on various aspects of this remarkable phenomenon.

carries a solar X-ray package (the “X-Ray Sensor”, or XRS) consisting of a collimator that feeds a pair of ion chambers. These ion chambers measure the Sun’s spatially integrated soft X-ray flux in two wavelength bands, 0.5–4 and 1–8 with a 3-s cadence. The GOES soft X-ray detectors have provided an essentially uninterrupted monitor of the Sun’s activity for 30 years, and are a valuable re- source for the study of past solar activity and the prediction of space weather (e.g., Bornmann, 1990; Phillips and Feldman, 1995; Aschwanden and Alexander, 2001; Garcia, 2004). For quantitative physical understanding of processes in the Sun’s atmosphere,

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Observations

Figure 1 shows 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.

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.

Figure 3: RHESSI 25 to 300 keV and GOES time derivative light curves for the flare starting at 01:52 UT on 23 October 2013 (SOL2013-10-23)

The right panel of Figure 1 shows .

Heading 2

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.

Conclusion

Small features in the GOES light curves are real even well into the decay phase of the flare.

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"