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Asymmetric Ionospheric Outflow

Dependent Source of Asymmetry

Relative Importance - Secondary


Description:

The rate of ionospheric outflow of light and heavy ions can be highly asymmetric between hemispheres.  This occurs both on a large scale (e.g., the total escaping ion fluence per species is hemisphere-dependent) and local scales (outflow features observed in one hemisphere are not observed at the conjugate location). 

The most well-known source of asymmetric outflow is differences in sunlight coverage due to season and time of day.  The hemisphere receiving more sunlight, and therefore extreme ultraviolet (EUV) flux, will have greater photoionization at ionospheric altitudes.  This increases the seed population for upflows and ultimately outflows.  Because outflow is driven by a variety of factors (see reviews by Hultqvist et al., 1999; Welling et al., 2015), all of which may be asymmetric themselves, the interhemispheric variance of ionospheric outflow can intensify during active periods.

Barakat et al. [2015] illustrates this effect clearly.  Figure 1 shows the total H+ (top frame) and O+ (bottom frame) outflow rate integrated over the northern (red lines) and southern (blue lines) hemispheres during a geomagnetic storm occurring from Sept. 29 to Oct. 4, 2002.  Values are obtained via the Generalized Polar Wind (GPW) model.  A clear diurnal pattern arises, favoring the southern hemisphere due the fact that it is post-equinox/southern hemispheric springtime.   Geomagnetic activity begins to intensify, as signified by the Kp index.  As activity increases, the total amount of outflow increases, as does north-south asymmetries.

 

 

The effects of asymmetric ionospheric outflow are not well understood.  There are implications for the ionospheric supply of magnetospheric plasma, which feeds the plasma sheet and ring current.  If and how an imbalance of outflow between the northern and southern hemisphere changes this supply is yet to be explored.

 

Computer simulations performed by CUSIA have found that asymmetric outflow can change the tilt of the magnetotail beyond what is expected due to dipole tilt. Simulations that include this effect have better agreement with in-situ observations.  Changing the magnetic field geometry of the magnetosphere will have trickle-down effects on ring current dynamics, precipitation, and more.  However, such effects are largely unexplored.

Modeling Capability:

Models are largely capable of capturing these effects, but care must be taken to ensure that the configuration enables it.  Global MHD models include outflow through setting inner boundary conditions, either with arbitrary values or via values obtained from empirical or first-principles-based outflow models.  The default inner boundary conditions of LFM do not allow any ionospheric outflow.  While BATS-R-US’ default boundary conditions can drive outflow into the magnetosphere (Welling & Liemohn, 2014), it is unclear to what extent the outflow is asymmetric.  The major physics-based outflow models (PWOM, GPW, IPWM) can all be configured to simulate both hemispheres.  However, it may not always be the default behavior, so model users must be aware.  For example, early PWOM – BATS-R-US coupling mirrored the northern hemisphere to the southern (Glocer et al., 2009) despite having the capability to handle both hemispheres (e.g., Glocer et al., 2020). 

BATS-R-US MHD


Yes, via coupling to other models

LFM MHD


Yes, via coupling to other models

PWOM


Can capture asymmetric outflow

IPWM


Can capture asymmetric outflow

GPW


Can capture asymmetric outflow


Yes, via coupling to other models

References:

Hultqvist et al., 1999 10.1007/978-94-011-4477-3 

Welling et al., 2015: 10.1007/s11214-015-0187-2 

Barakat et al., 2015: 10.1002/2015GL065736 

Welling & Liemohn: 10.1002/2013JA019374 

Glocer 2009: 10.1029/2009JA014418 

Glocer 2020: 10.1029/2020ja028205 

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