Investigation of the multiple‐component structure of the 20 January 2005 cosmic ray ground level enhancement

Abstract

[1] Worldwide observations of the cosmic ray ground level enhancement (GLE) of 20 January 2005 are used to investigate a commonly observed but poorly understood feature of this class of event. It is argued that the GLE comprised two distinctly different cosmic ray populations. The first resulted in an impulsive, highly anisotropic, field-aligned pulse with a relatively hard rigidity spectrum and significant velocity dispersion. The characteristics of the anisotropy were almost identical to those for similar impulsive increases observed during GLEs in 1960GLEs in , 1978GLEs in , and 1989. The p 0 g ray observations from the RHESSI and CORONAS-F spacecraft and Type III radio emissions yield a path length of 1.76 ± 0.1 AU to Earth for the first pulse. After the highest energies in the initial anisotropic pulse had passed Earth, another field-aligned but mildly anisotropic cosmic ray pulse developed slowly worldwide, exhibiting the characteristics of the conventional GLE. The risetime and anisotropy of this second population indicate substantial scattering, apparently at variance to the essentially scatter-free nature of the initial pulse. We show that the coexisting scatter-free initial impulsive increase and the diffusive character of the second pulse are consistent with the standard quasi-linear theory of pitch angle diffusion. Throughout the GLE, the anisotropy remained field-aligned, and a third maximum, seen by some stations, is shown to be due to changes in the direction of the heliospheric magnetic field (HMF). Examination of 22 large (>20%) GLEs in the historical record shows that the impulsive pulse never occurs after the commencement of the P2 pulse, indicating that the impulsive-gradual combination is not due to a chance sampling of differing scattering regions of the HMF. It is further shown that impulsive pulses, or their equivalents, have been observed in 13 out of the 15 GLEs associated with solar activity in the solar longitude range 24°-98°W, leading us to propose that the event of 20 January 2005 should be regarded as the defining example of the GLE. The observations lead us to propose two separate acceleration episodes in the typical GLE: (1) acceleration directly associated with the flare itself and located in the lower corona and (2) acceleration by a supercritical shock driven by the associated coronal mass ejection, located at $3-5 solar radii and farther in the upper corona. A one-to-one association with so-called impulsive and gradual solar energetic particle events at lower energies is proposed. On the basis of these observations, a generic model for the GLE is proposed.

The relativistic solar particle event of 2005 January 20: origin of delayed particle acceleration

“…Results for the relativistic SEP event of 2005 Jan. 20 presented in Paper I and the present work are consistent with the basic scheme of successive relativistic particle releases during a GLE devised by Miroshnichenko (2001, and references therein), Vashenyuk et al (2006), and McCracken et al (2008. Our observations relate the acceleration processes to evolving large-scale magnetic structures in the corona and therefore to the CME development.…”

“…It displayed a well-defined, rapidly rising time profile at the beginning and a distinct second peak a few minutes later. Evidence that the first release was related to particle acceleration in the flaring active region in the low corona was brought by different publications (e.g., Simnett 2006Simnett , 2007Kuznetsov et al 2008;Grechnev et al 2008;McCracken et al 2008;Masson et al 2009). In the present paper we complete the investigation of Masson et al (2009, hereafter Paper I) through a detailed comparison between the second peak of the relativistic proton time profile derived from neutron monitor measurements with high-quality radio and UV observations.…”

What Are Special About Ground-Level Events?

“…Whether a given SEP event is a GLE depends on the detection of the secondary particles usually by neutron monitors (NMs) at cosmic ray stations. For a given GLE, different NMs show different time profiles (e.g., Bieber et al 2002;McCracken et al 2008;Moraal and McCracken 2011), because of different asymptotic look angles and rigidities of the NMs combined with anisotropy of the particles.…”

“…Another view, however, is that some GLEs come directly from solar flares, which may also accelerate particles to high energies in magnetic reconnection, primarily because their temporal variations mimic those of impulsive flares (Grechnev et al 2008;McCracken et al 2008). Aschwanden (2012) also argued that, in five of the 13 GLEs he studied, the particles are released during the impulsive phase of the associated flare.…”

Two solar proton fluence models based on ground level enhancement observations

“…Another distinguishing factor is the particle composition: impulsive events are characterized as electron-and 3 He-rich and with enhanced abundances of heavy ions (e.g., Reames et al, 1985;Reames, 1988), whereas the particle composition of gradual events more or less resembles that of the solar corona (e. g., Meyer, 1985). Although GLEs are usually considered as extreme examples of gradual events, some studies suggest that GLEs have a direct flare component, based on their temporal behaviour (Grechnev et al, 2008;McCracken et al, 2008), or particle composition and charge states (Cohen et al, 1999;Mason et al, 1999;Möbius et al, 1999). It has also been suggested that the energy-dependent composition and charge states in these events can be understood in terms of evolving shock-normal angle in a shock acting on remnant flare suprathermals from earlier flare activity (Tylka et al, 2005;Tylka and Lee, 2006;Sandroos and Vainio, 2007;Sandroos and Vainio, 2009).…”