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The Carrington Event: Possible Solar Proton Intensity–Time Profile

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The Carrington Event: Possible Solar Proton Intensity–Time Profile Advances in Space Research 38 (2006) 215–225 www.elsevier.com/locate/asr The Carrington event: Possible solar proton intensity–time profile D.F. Smart a,*, M.A. Shea a, K.G. McCracken b a Emeritus at Air Force Research Laboratory (VSBX), Hanscom AFB, Bedford, MA 01731-3010, USA b IPST, University of Maryland, College Park, MD 29742-2431, USA Received 9 November 2004; received in revised form 5 April 2005; accepted 5 April 2005 Abstract A possible >30 MeV solar proton intensity–time profile associated with the Carrington solar flare event of 1 September 1859 is constructed. The derived profile is consistent with a >30 MeV proton omni-directional fluence of 1.9 · 1010 cmÀ2 found by the anal- ysis of solar proton generated NOy radicals that are deposited in polar ice. The intensity–time profile of the solar particle flux is constructed by assuming that the Carrington solar event is part of the class of interplanetary shock-dominated events where the maximum particle flux is observed as the shock passes the Earth. This assumption is based on the knowledge that the very large solar proton fluence events (those with >30 MeV omni-directional fluence exceeding 1.0 · 109 cmÀ2) associated with central merid- ian solar activity during the last 50 years belong to this class of event. The absence of a statistically significant increase in the observed concentration of the cosmogenic nuclide 10Be for 1859 indicates that the solar cosmic radiation produced in the Carrington event had a soft spectrum, similar to other interplanetary shock-dominated events. Ó 2006 Published by Elsevier Ltd on behalf of COSPAR. Keywords: Carrington event; Solar protons; Protons intensity; Proton time profile 1. Introduction stitute one class of events; we will call these ‘‘near-sun injection source events’’. The second class of events is 1.1. Characteristics of large fluence solar proton events associated with solar activity near the central meridian observed during the ‘‘space era’’ of the sun. We will refer to these events as ‘‘interplane- tary shock dominated events’’. For this second class of When the very large fluence solar proton events ob- events, a fast interplanetary shock from activity near served during the ‘‘space era’’ (1965 to present) are orga- the central meridian of the sun continuously accelerates nized by the helio-longitude of the associated solar active ions throughout its entire passage from the sun to the region, we find a distinct bi-modal distribution as illus- Earth. In this class of event, the maximum flux is ob- trated in Fig. 1. The events identified in this figure have served as the shock passes the observer. a >30 MeV omni-directional fluence exceeding A conceptual illustration of these two classes of solar 1 · 109 cmÀ2 as measured by spacecraft at 1 AU. This proton events is given in Fig. 2. For the ‘‘near-sun injec- bi-modal distribution of the very large fluence events per- tion source events’’ there is identifiable solar activity, sists as far back in time as we can confidently make the usually a significant solar flare with an associated fast solar flare activity–solar proton event time association. coronal mass ejection (CME) that is directed in the west- In this bi-modal distribution, the classic western ward direction as viewed from the Earth. This is illus- hemisphere solar activity associated proton events con- trated on the right side of Fig. 2. These particles are observable at the Earth as long as there is a good inter- * Corresponding author. Fax: +1 603 888 4733. planetary magnetic field connection between the particle E-mail address: [email protected] (D.F. Smart). acceleration region and the Earth. Since the classic 0273-1177/$30 Ó 2006 Published by Elsevier Ltd on behalf of COSPAR. doi:10.1016/j.asr.2005.04.116 216 D.F. Smart et al. / Advances in Space Research 38 (2006) 215–225 CME continuously accelerates particles as it propagates away from the Sun toward the Earth as illustrated on the left side of Fig. 2. The solar particle fluence at the Earth is the integral of the particles generated by the shock and propagated along the interplanetary magnetic field lines connecting the shock to the Earth. In the eight central meridian events identified in Fig. 1, the maximum flux was observed as the shock passed the Earth-orbiting satellite that acquired the solar proton measurements and this generally corresponded to a major geomagnetic storm commencement. 2. Historical large fluence solar proton events We have searched the published literature to ascertain Fig. 1. The helio-longitudinal distribution of very large fluence that the bi-modal distribution shown in Fig. 1 is a system- (1 · 109 cmÀ2) >30 MeV solar proton events observed at 1 AU during atic effect present in all known large fluence solar proton the spacecraft measurement era (1965-present). The top number in events. As shown in Fig. 3, this bi-modal pattern does not each square indicates the omni-directional fluence; the bottom number change when the large solar proton events of the 18th and indicates the year, month and day of the event. The dates represent 19th solar cycles are combined with the events of the either the start of a solar activity episode or the dominant event in a solar activity episode. spacecraft era. For the events in the 19th solar cycle we used the fluence values from Shea and Smart (1990); for Archimedean spiral path from the Sun to the Earth is the events in the 18th solar cycle we used the >30 MeV ‘‘rooted’’ between 50° and 90° West helio-longitude proton fluence derived from the nitrate (NOy) deposits (depending on the solar wind speed), the connection to in polar ice (specifically the GISP2-H core from central the strong particle acceleration source is relatively brief Greenland) as described by McCracken et al. (2001a). (due to the fast westward motion of the CME/shock) We note that while there are some uncertainties about and consequently, the time of intense particle flux obser- the magnitude of the solar proton fluence values derived vations at 1 AU is correspondingly brief. There is gener- from the NOy deposits, the uncertainties are not signifi- ally little subsequent associated geomagnetic activity. cant with respect to the bi-modal distribution. For the ‘‘interplanetary shock dominated events’’, the Solar cosmic ray ground-level events (GLEs) observed governing feature is a fast broad coronal mass ejection as a consequence of solar activity at helio-latitudes from solar activity near the central meridian of the >60°W (i.e., in the western member of the bimodal distri- sun. The fast interplanetary shock forward of this bution) generally exhibit a significantly harder spectrum Fig. 2. Conceptual illustration of the two classes of large fluence solar particle events. Upper left: the ‘‘interplanetary shock dominated event’’ results from solar activity near the central meridian of the sun. The CME shock continuously accelerates particles during its transit from the Sun to the Earth. Upper right: the ‘‘near-sun injection source event’’ dominates the observed flux at the Earth only as long as there is a good interplanetary field connection between the accelerating source region and the Earth. The lower panels illustrate typical intensity–time profiles of particles at 1 AU corresponding to each class of event. D.F. Smart et al. / Advances in Space Research 38 (2006) 215–225 217 EarthÕs upper atmosphere generating the nitrate radicals and subsequent ozone depletion are complex (Jackman et al., 1990, 1995) and a detailed description is beyond the scope of this paper. (The NOy consists of N, NO, NO2,NO3,HN2O5,N2O5, HNO3,HO2NO2, ClONO2 and BrONO2.) Some of the produced HNO3 is trans- ported to the troposphere, where it is generally precipi- tated to the surface. Measurements of impulsive nitrate deposition in polar ice are markers of the HNO3 precipitation. This nitrate radical generation and consequent ozone depletion has been observed for every major solar pro- ton event (>30 MeV fluence >109 cmÀ2) of the space era. There exists a relatively large background of terres- trial sources of NOy (see Jackman et al., 1980, 1990, 2000), and only very large fluence solar proton events (those with a >30 MeV omni-directional fluence exceed- ing 1 · 109 cmÀ2) will generate sufficient NOy to be observable above this terrestrial background. The exper- imental evidence from high-resolution sampling of polar ice cores indicates that the deposition of these NOy Fig. 3. The helio-longitudinal distribution for all very large fluence 9 2 radicals in polar ice begins 6 weeks after the initiating (1 · 10 cmÀ ) >30 MeV solar proton events observed at 1 AU during solar cycles (SC) 18 through 23. The top number in each square is the solar proton event (Zeller and Dreschhoff, 1995; Dresch- solar sunspot cycle number; the middle number in each square hoff and Zeller, 1990, 1998; McCracken et al., 2001a). indicates the omni-directional fluence. The bottom number in each This association between the impulsive NOy depositions square is the date of the event. The dates represent either the start of a found in polar ice and exceptional solar proton events solar activity episode or the dominant event in a solar activity episode. has been extended back to the beginning of routine cos- Solar cycle 18 began in 1944.2; solar cycle 23 began in 1996.8 and is in its late decaying phase in 2005. mic radiation measurements in the 1930s and to excep- tional solar–terresterial/major geomagnetic storm than those associated with solar activity near the central events since 1909 (McCracken et al., 2001a; Shea meridian.
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