Major Flare Watch Evaluation

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Number: 140
1st Author: Dick Canfield
2nd Author: Shaun Bloomfield
Published: 15 November 2010
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High-energy X- and gamma-ray emissions are the most direct radiative signatures of energetic particles during solar flares. While RHESSI provides high-resolution imaging spectroscopy of these emissions, observations by other space- and ground-based instruments provide crucial information on the context in which these high-energy emissions are generated. Some of these other instruments have a limited field-of-view. Some have modes optimized for major flares. The SoHO Hotshot "All Together Now: X1.5" [1] is a great demonstration of how coordination of observations benefits one and all.

Maximization of the scientific return of solar flare data is the over-arching purpose of the various elements of the Max Millennium Program of Solar Flare Research [2]. Daily email alerts are issued by one of the Max Millennium Chief Observers (MM_COs)[3]. Such announcements appear in the Max Millennium Messages of the Day [4], distributed worldwide to over 200 solar observers and mission operations planners through the self-subscribing MMmotd mailing list and archive [5].

Major flares that produce gamma-ray emission are the highest priority target of the RHESSI Mission. As such, multi-wavelength imaging and spectroscopic observations from other spacecraft and ground-based observatories before, during and after such flares are crucial to the interpretation of RHESSI data from these events. If a region is likely to produce a major event, the MM_COs declare that a Major Flare Watch campaign is in effect. How effective have these campaigns been over the course of the RHESSI mission? That's the subject of this Nugget.

Major Flare Watch Criteria

Throughout the RHESSI mission, the MM_COs have used a set of Major Flare Watch criteria that are an outgrowth of the classic "BEARALERTS - A Successful Flare Prediction System" paper by Zirin and Marquette (1991)[6]:

  1. A major flare has occured (if a region produced one big flare, it will probably produce at least one more)
  2. Large island delta - opposite polarity umbrae within 2 heliographic degrees within a common penumbra (delta configuration) and, in addition, the delta surrounded by opposite polarity flux - even better if the region is reversed polarity. Bright H-alpha will be present.
  3. Large delta configuration with bright H-alpha plage and better still if reversed polarity. Bright H-alpha along the neutral line is needed.
  4. Elongated umbrae in pairs of opposite polarity even if the umbrae are not a delta configuration. Transverse magnetograms will reveal strong shear.
  5. Emerging Flux Region (EFR) within an existing active region, if the leader spots of the EFR are adjacent to the existing region's trailing spots or vice-versa.
  6. Rapidly moving sunspots - sunspot(s) moving towards and/or into an opposite polarity spot.

Statistics for Daily Major Flare Watch Messages

We've had a look at all GOES X-ray flares recorded in the daily NOAA/NWS Space Weather Prediction Center events files from 26-Mar-2001 to 17-May-2010, inclusive. We extracted the number of flares at or above a chosen GOES class within 24 hours of the Major Flare Watch UTC message issue times. The total number of flares at or above the same GOES class is extracted from the complete time range under consideration (26-Mar-2001 to 17-May-2010) allowing the percentage to be calculated. We also extract the number of flares found within each of the 220 MFW 24 hour periods, so the number of MFW messages that both successfully and unsuccessfully catch flares is extracted and the percentage of MFW messages which are successful is calculated.

Table 1: Number of flares at or above a chosen GOES class within 24 hours of the Major Flare Watch UTC message issue times (24-hour MFW periods, MFWs below) during the period 26-Mar-2001 to 17-May-2010

In graphical form (Figure 1), these results show that the percentage of all flares caught in MFWs by the MM_COs saturates at about 75% in the X-class range, which is roughly coincident with the occurence of gamma-ray lines observed by RHESSI (Shih et al., 2009) [7]

Figure 1: Percentage of flares at or above a chosen GOES class within 24 hours of the Major Flare Watch UTC message issue times (24-hour MFW periods, aka MFWs) for 26-Mar-2001 to 17-May-2010

What if MWF Periods Started/Ended Earlier or Later?

For practical reasons, it may be difficult for any given observatory or spacecraft to respond immediately to the declaration of a Major Flare Watch. To consider the effect of delayed response, we identified all 33 periods of contiguous daily MFW messages between 1-Feb-2001 to 31-May-2010. We then used the NOAA/NWS data to ask "What if these MFW periods had started one day earlier/later or ended one day earlier/later? Figure 2 shows that success catching > X1 flares is independent of 1-day delays in MFW period start dates. Of course, if we could get a crystal ball, we would, so that we never had to implement the all-important first criterion in the list above.

Figure 2: Percentage of flares at or above a chosen GOES class within 24 hours of the Major Flare Watch UTC message issue times (24-hour MFW periods, aka MFWs) for 26-Mar-2001 to 17-May-2010, for various offsets in time of the start/end of the MFW period


Our analysis shows that, over the course of Solar Cycle 23, about 75% of all flares of X1-class or greater were caught during 220 days of MFW campaigns. That amounts to less than 7% of all observing days during that period and is independent of a 1-day delay in calling a MFW campaign. Pretty cost effective, don't you think?!

A new compliment of space- and ground-based instruments is poised for the increased activity expected during Solar Cycle 24. The Max Millennium team aims to continue its service to the solar flare community via continued daily email alerts on solar activity, maintenance of the Solar Physics ePrint Archive, and coordination with mission planners and ground based observers to optimize the scientific return on their data.

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