Forecasting Solar Energetic Proton events (E

A Solar proton event occurs when protons emitted by the Sun become accelerated to very high energies during a solar flare accompanied by a coronal mass ejection or in interplanetary space by the shocks associated with coronal mass ejections. Protons are finally guided by the interplanetary magnetic field lines.

Energetic proton storms can electrically charge spacecraft to levels that can damage electronic components. Solid state memory can be altered.

Energetic solar protons are a significant radiation hazard to spacecraft and astronauts. They can cause spacecraft to lose their orientation. Flashes and streaks of light occur when energetic protons strike the sensitive optical electronics in spacecraft and can also destroy the efficiency of the solar panels. Significant proton radiation exposure can be experienced by astronauts who are outside a protective shield in the space.

Earth is largely protected by its magnetic field, or magnetosphere. However, the closer a spacecraft (or aircraft) approaches the polar regions, the greater the exposure to energetic proton radiation will be.

Depending upon the observer’s longitude relative to the originating solar event, several intensity profiles that range from magnetically well-connected to poorly-connected solar energetic proton (SEP) events are possible [Reames, 2004]:

Fig. 1. Solar energetic protons emitted by the Sun become accelerated to very high energies. Energetic protons and ions are finally guided by the interplanetary magnetic field lines.

If the satellite, situated near Earth (at 1 AU), is magnetically well connected with the originating solar event, it may observe rapidly rising solar proton intensities (E>10 MeV). The particles in these SEP events are accelerated by flares and coronal mass ejections (CMEs). Mostly associated with western (right-hand side of the Sun) events, well-connected SEP events may also be associated with solar eastern events.
If the satellite, situated at 1 AU, is magnetically poorly connected with the originating solar event, it may observe a slow increment in proton intensity (E>10 MeV), surpassing the SEP threshold 1-3 days after the solar event. The particles in these SEP events are accelerated by shock waves driven out from the Sun by CMEs. The observer witnesses a maximum intensity only after crossing through the shock into the region where field lines connect to the shock nose from behind.

The UMASEP forecaster

UMASEP [Núñez, 2011] is a system that forecasts SEP events (E>10 MeV) in real-time. This system is based on a dual-model approach for predicting the time interval within which the integral proton flux is expected to meet or surpass the Space Weather Prediction Center threshold of J (E >10 MeV) = 10 pr cm^-2 sr^-1 s^-1 and the intensity of the first hours of well- and poorly-connected SEP events. The purpose of the first model is to identify precursors of well-connected events by empirically estimating the magnetic connectivity from the associated CME/flare process zone to the near-Earth environment and identifying the flare temporally associated with the phenomenon. This model analyzes flare and near-Earth space environment data (soft X-ray, differential and integral proton fluxes). The goal of the second model is to identify precursors of poorly-connected events by using a regression model that checks whether the differential proton flux behavior is similar to that in the beginning phases of previous historically poorly-connected SEP events, and thus deduce similar consequences. An additional module applies a higher-level analysis for inferring additional information about the situation, by filtering out inconsistent preliminary forecasts and estimating the intensity of the first hours of the predicted SEP events. The high-level module periodically retrieves solar data and, in the case of well-connected events, automatically identifies the associated flare and active region.

Real-time UMASEP forecasts are publicly presented in three space weather sites: NASA's iSWA system [Madox et al 2008], European Space Weather Portal, and UMASEP forecast panel. These forecasts may also be consulted by ftp or by using the SOAP Web Service UMASEPForecasterService.

The graphical output of the UMASEP Forecaster is updated automatically in the forecast panel (http://spaceweather.uma.es/forecastpanel.htm). Figure 3 shows the forecast panel that an operator would have seen if the current UMASEP Forecaster had processed the real-time GOES data during Oct 26th, 2003. This figure also shows inferences about the associated flare, heliolongitude and active region, as well as a small illustration of a possible route of the solar protons from the corresponding heliolongitude toward the near-Earth environment.

The upper time series shows the integral proton flux with energies greater than 10 MeV. The current flux is indicated below the label "now". To the right of this label, the forecasted integral proton flux is presented. Colors indicate the intensity of the expected integral proton flux at that specific time.

The middle time series shows recent solar activity in terms of soft X- rays and the lower time- series shows the magnetic connectivity with the most well-connected flaring region. When a forecast is issued, the graphical output also shows the details of these predictions and what the model infers about the situation. Figure 2 shows the output of the forecaster after processing the data of October 2003. At 18:00 on October 26th, the integral flux can be seen to be almost flat, but the forecaster issues a prediction. The forecast details showed that the expected storm would arrive during the following 2 hours and it would reach intensity in the range from two hundred to three hundred pfu. Below the forecast section, the system also presents the model inference section, which shows that the Earth is well-connected with the solar region 484, in which a solar flare has erupted. The system also shows that the associated heliolongitude is west 38. On the lower-right of the forecast panel, the system also presents a graphical illustration of the Sun-Earth link, showing a possible trajectory of the predicted protons. This illustration is useful to show that no solar proton storm has arrived to Earth but one is coming through the magnetic field lines.

Fig. 2. This figure shows the UMASEP output after processing GOES-10 data from October 26, 2003. The small upper-right chart is not part of the forecaster output; it shows the posterior evolution of the integral proton flux for this event, the first “Halloween” SEP event, showing that the forecast was successful. Note that the well-connected SEP was forecasted when no enhancement in the integral proton flux (E> 10 MeV) was observed because the correlated rise of proton channel (GOES-10 P6) occurred with low flux (as well as the rest of the differential proton channels).

Figure 3 shows the output of the forecaster for several poorly-connected SEPs. Note that the anticipation times of the storms of Figure 3 is are greater than 20 hours..

Fig. 3. This figure shows several successful forecasts of poorly-connected SEP events. This figure shows the forecast of two SEPs during December 7th, 2006 (upper chart) and April 16th, 1990 (lower chart). The respective anticipation times were 20:05 hours and 22:05 hours.

Validation results

For the SEP events from January 1994 until June, 2013, of the NOAA/SWPC SEP list, the UMASEP v1.2 has a probability of detection (POD) of all well- and poorly-connected events of 86% (104/121) and a false alarm rate (FAR) of 21.8% (29/133)¹,which outperforms current automatic forecasters in predicting >10 MeV SEP events. The presented forecaster has an average warning time (AWT) of 4 h 1 min for the successfully predicted events, 1 h 6 min for well-connected events and 8 h 9 min for poorly connected events, with a maximum warning time of 24 h for very gradual SEP events.

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¹ Note: For the SEP events before 1994, the GOES satelite's X-ray and proton observations included periods of high noise and data gaps, which increments the number of UMASEP's false alarms. Since the GOES satelite's data quality has improved, the UMASEP's validation results for those early years are not representative of current situations. Maybe for this reason, the validation of the recent SEP forecasters do not include GOES's data before 1994. Just for information purposes, we inform that for the period 1986-2013 the POD of UMASEP is 82.72% (158/191), the FAR is 31.3%, (72/230), and an AWT is 4 h 49 min.

References

Núñez, M. (2011), Predicting solar energetic proton events (E > 10 MeV), Space Weather, 9, S07003, doi:10.1029/2010SW000640. Preprint (PDF)

Maddox, M., M. Hesse, M. Kuznetsova, M. Rastaetter, L. MacNeice, P. J. Jain, W. Garneau, D.H. Berrios, A. Pulkinnen, and D. Rowland (2008), The Integrated Space Weather Analysis System, paper presented at the American Geophysical Union Fall Meeting 2008, Website: http://iswa.gsfc.nasa.gov/iswa/iSWA.html
Reames, D. V. (2004), Solar Energetic Particle Variations, Adv. Space Res. 34 (No. 2), 381.