Models

SAMI3

Sami3 is A Model of the Ionosphere

SAMI3 (Sami3 is Also a Model of the Ionosphere) is a seamless, global, three-dimensional, physics-based model of the ionosphere/plasmasphere system (Huba et al., 2000, 2008; Huba&Sazykin 2014). SAMI3 models the plasma and chemical evolution of seven ion species (H+, He+, N+, O+, N+2, NO+ and O+2). The temperature equation is solved for three ion species (H+, He+ and O+) and for the electrons. Ion inertia is included in the ion momentum equation for motion along the geomagnetic field. This is important in modeling the topside ionosphere and plasmasphere where the plasma transitions from collisional to collisionless. The neutral composition, temperature, and winds can be specified in SAMI3 by analytical models, empirical models (NRLMSISE00, HWM93, HWM14) or, as planned in the CGS, by the physics-based models (e.g., WACCM-X). SAMI3 nominally uses the EUVAC [Richards et al., 1994] model for solar radiation. However, it can also use the FISM EUV model [Chamberlin et al., 2008]. In the MAGE model SAMI3 will play a dual role: it will serve as a plasmasphere component within GAMERA and as an ionosphere component for WACCM-X, which will, in turn, provide it with the neutral composition, temperature, and winds.

The electrostatic potential used in SAMI3 in the low- to mid-latitude ionosphere is determined by the solution of a potential equation driven by the neutral wind dynamo [Huba et al., 2008]. This equation is based on two assumptions: ∇⋅J=0 and equipotential field lines. As part of MAGE, SAMI3 will received the potential in the high-latitude region from the REMIX module. SAMI3 uses a nonorthogonal, nonuniform, fixed grid. The grid is designed to optimize the numerical mesh so that the spatial resolution decreases with increasing altitude. The plasma is transported transverse to the geomagnetic field using a finite volume method in conjunction with the donor cell method [Huba, 2003]. The model uses an aligned or tilted magnetic dipole field, and has recently been extended to use the Richmond apex model [Richmond, 1995].

SAMI3 has been used to explore a wide-variety phenomena concerning the ionosphere-plasmasphere system: equatorial spread F irregularities [Huba and Joyce, 2010; Huba and Krall, 2013b; Huba and Liu, 2020], solar eclipses [Huba and Drob, 2017], impact of geomagnetic storms on the low- to mid-latitude ionosphere [Huba and Sazykin, 2014; Huba et al., 2016], metal ion layer dynamics [Huba et al., 2019, 2020; Krall et al., 2020], and plasmasphere dynamics [Huba and Krall, 2013a].

As an example of recent work regarding equatorial spread F, Fig. 1 shows results from a high-resolution SAMI3/WACCM-X simulation [Huba and Liu, 2020]. The Fig. 1 compares 135.6 nm emissions from the simulation for the March case at 23:59 UT (left and middle panels) to GOLD emission observations (right panel) from geosynchronous orbit [Eastes et al., 2019]. The GOLD results are for October, 2018 which corresponds to equinox conditions at solar minimum, similar to the conditions of the simulation. The center panel is on the same color scale as the GOLD data (maximum of 40 Rayleighs); it shows that the intensity of the 135.6 nm emissions from the model is less than the data. The left panel reduces the color scale maximum to 12 Rayleighs in order to highlight the structure in the model results; it shows a remarkable similarity to the data. The model results capture the extended ionization arcs from the post-sunset period (eastern South America) to midnight (western Africa) observed in the data. Moreover, regular plasma striations (bubbles) are also observed in the model as in the data on similar scale lengths.

Figure 1: Comparison of 135.6 nm emissions from the simulation for the March case (left and middle panels) and GOLD emission data (right panel) observed from geosynchronous orbit [Eastes et al., 2019].