Neutralized Solar Energetic Particles for SEP Forecasting: Feasibility Study of an Innovative Technique for Space Weather Applications

Abstract

Energetic neutral atoms (ENAs) are produced by the neutralization of energetic ions formed by shock-accelerated gradual solar energetic particle events (SEP). These high-energy ENAs (HENAs) can reach the Earth earlier than the associated SEPs and provide information about the SEPs at the lower corona. The HENA properties observed at Earth depend on the properties of the coronal mass ejection (CME)-driven shocks that accelerate the SEPs. With a model of HENA production in a shock-accelerated SEP event, we semi-quantitatively investigate the energy-time spectrum of HENAs depending on the width, propagation speed, and direction of the shock, as well as the density and ion abundances of the lower corona.Compared to the baseline model parameters, the cases with a wider shock width angle or a higher coronal density would increase the HENA flux observed at the Earth, while the case with an Earth-propagating shock shows a softened HENA spectrum. The comparison of expected HENA fluxes in different cases with a flight-proven ENA instrument suggests that solar HENAs can feasibly be monitored with current technologies, which could provide a lead time of 2-3 hours for SEPs at a few MeV. We propose that monitoring of solar HENAs could provide a new method to forecast shock-driven SEP events that could have significant space weather impacts on the near-Earth environment.

Modeling Solar Energetic Neutral Atoms from Solar Flares and CME-driven Shocks

“…The first ENA simulation was done by Wang et al (2014), who simulated a CME-driven shock from a side-on orientation and suggested that the observed flux in Mewaldt et al (2009) is consistent with ENA production at a CME-driven shock. More recently, Wang et al (2014Wang et al ( , 2022 examined a variety cases with different CME speeds, open angles, and CME propagation directions. They also examined the effect of solar wind density variation near the Sun on the production of ENAs.…”

“…The energy ranges for these cross sections are also limited. Following Wang et al (2022), we have extended them to a larger energy range whenever necessary. It should be noted that, as in Wang et al (2014), we ignore charge exchanges by other ions (He + , N 5+ , etc.…”

Energetic Neutral Atoms from Solar Energetic Particles due to Shocks: Inclusion of Upstream Particles

“…As in W14, we presume the center of the shock has a trajectory in the ecliptic plane and is perpendicular to the Earth-Sun line. The shape of the shocked region is conical, with a spherical cap at the position of the shock, and a spherical indent at 1.5 R e (see Figure 1 of Wang et al 2022). The full opening angle of the cone is 60°, again, as in W14.…”

“…where Θ(ξ) is the Heaviside step function; r is the heliocentric distance and is equal to + + x y z 2 2 2 ; r s is the shock's location and is equal to r s,0 + u s t; the initial shock location r s,0 is equal to 1.5 R e , as in W14; f d is the downstream distribution and is simply equal to f 0 ; n(r s ) is the solar wind density at the location of the shock and is calculated using Equation (2) in W14; n 0 is the density at which the shock simulations were run (see Section 3); and the cone angle of the shock is defined by y = y r cos . In the above, we have used the same coordinate system as Figure 1 of Wang et al (2022), where the Sun is at the origin, the observer is at 1 au in the positive x-direction, and the CME's trajectory is in the positive y-direction.…”

“…There has been recent interest in the potential of energetic neutral atoms (ENAs) as remote probes of solar energetic particles (SEPs) and their acceleration. The single observation that has been reported, at least in physical units (Mewaldt et al 2009)-there have been recent exciting results reported using SAMPEX data, but these are only shown in terms of instrument count rates (Mason et al 2021)-has been modeled as accelerated by a coronal mass ejection (CME)-driven shock (Wang et al 2014;W14 hereafter), which was recently enhanced by a parameter study using a similar model (Wang et al 2022), in which they varied the CME speed, direction, and opening angle, as well as extended to include flare acceleration (Li et al 2023). One of the assumptions common to all these studies is that the upstream component of the shock can be ignored.…”