The Impact of Multifluid Effects in the Solar Chromosphere on the Ponderomotive Force under SE and NEQ Ionization Conditions

Abstract

The ponderomotive force has been suggested to be the main mechanism to produce the so-called first ionization potential (FIP) effect—the enrichment of low-FIP elements observed in the outer solar atmosphere, in the solar wind, and in solar energetic events. It is well known that the ionization of these elements occurs within the chromosphere. Therefore, this phenomenon is intimately tied to the plasma state in the chromosphere and the corona. For this study, we combine IRIS observations, a single-fluid 2.5D radiative magnetohydrodynamics (MHD) model of the solar atmosphere, including ion–neutral interaction effects and nonequilibrium (NEQ) ionization effects, and a novel multifluid multispecies numerical model (based on the Ebysus code). Nonthermal velocities of Si iv measured from IRIS spectra can provide an upper limit for the strength of any high-frequency Alfvén waves. With the single-fluid model, we investigate the possible impact of NEQ ionization within the region where the FIP may occur, as well as the plasma properties in those regions. These models suggest that regions with strongly enhanced network and type II spicules are possible sites of large ponderomotive forces. We use the plasma properties of the single-fluid MHD model and the IRIS observations to initialize our multifluid models to investigate the multifluid effects on the ponderomotive force associated with Alfvén waves. Our multifluid analysis reveals that collisions and NEQ ionization effects dramatically impact the behavior of the ponderomotive force in the chromosphere, and existing theories may need to be revisited.

Identifying Plasma Fractionation Processes in the Chromosphere Using IRIS

“…Throughout this time period, the leading polarity region has a consistently lower associated FIP bias than the following polarity region, where the unsigned flux density is consistently below 200 G. The emerging flux region also follows this hypothesis, with an FIP bias value between that of the leading and following polarity regions and a consistent unsigned flux density value of ∼50-100 G. These observations are also comparable with the work of To et al (2023), who found a similar relationship between coronal abundance and magnetic flux density using observations of this active region from 2020 April 3-7. It is interesting that this is also consistent with the work of Martínez-Sykora et al (2023), who suggested that sunspots with strong flux densities approach a collisionless case, where waves from the chromosphere could generate an IFIP effect, thus lowering the FIP bias associated with the region (as noted here) or even creating an observable IFIP bias.…”

“…In contrast, the emerging flux region exhibits a much broader range of values, typically peaking at a lower value than observed for the following polarity region (with the exception here of the C II line in panel (i). As noted by Martínez-Sykora et al (2023), the Si IV line width provides insight into unresolved velocity due to Alfvén waves, so we chose the Si IV 1403 Å line width for further analysis in Section 4.…”

“…In particular, the line width for the Si IV 1403 Å line was chosen for further analysis as there were significant differences in the observed behavior between the different regions of interest. This parameter has also been used by Martínez-Sykora et al (2023) to infer the presence of unresolved Alfvén waves. The turbulence velocity derived from the IRIS 2 inversions has previously been used by Testa et al (2023) to probe the fractionation process and relationship with FIP bias in outflow regions.…”

“…However, the fractionation process in the upper chromosphere and transition region has, to date, been underinvestigated. Dahlburg et al (2016) and Martínez-Sykora et al (2023) have begun the process of extending the ponderomotive force model initially proposed by Laming (2004Laming ( , 2009Laming ( , 2015 to include a multifluid analysis and nonequilibrium ionization effects, both of which are important in the solar chromosphere where this process should be occurring. Observations of this region provided by the Interface Region Imaging Spectrograph (IRIS; De Pontieu et al 2014) spacecraft are also being used to investigate this process, despite the lack of suitable emission lines for estimating FIP bias within the wavelength range probed by IRIS.…”

Intriguing Plasma Composition Pattern in a Solar Active Region: A Result of Nonresonant Alfvén Waves?

“…More recent numerical simulations by Réville et al (2021) using a shell turbulence model found that, under the assumption that turbulence is the main driver of coronal heating and solar wind acceleration, a ponderomotive force can appear in the chromosphere and the transition region, and can be strong enough to create the FIP effect. Martínez-Sykora et al (2023) use a combination of observations from the IRIS mission (De Pontieu et al 2014) and a 2.5D radiative magnetohydrodynamics model of the solar atmosphere to investigate the multifluid effects on the ponderomotive force associated with Alfvén waves.…”