HAO 2012 Profiles In Science: Dr. Ingrid Cnossen

In 2012 Dr. Ingrid Cnossen was a Post Doc II at the High Altitude Observatory at NCAR (now moved to the British Antarctic Survey). She studies the effects of long-term changes in the Earth's magnetic field on magnetosphere-ionosphere-thermosphere interactions and on the upper atmosphere itself, using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model.

Publications

Figure 1: High resolution

(1) Ingrid Cnossen, Arthur D. Richmond, and Michael Wiltberger, 2012: The dependence of the coupled magnetosphere-ionosphere-thermosphere system on the Earth's magnetic dipole moment.

Abstract: The strength of the Earth's magnetic field changes over time. We use simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere model to investigate how the magnetosphere, upper atmosphere, and solar quiet (Sq) geomagnetic variation respond as the geomagnetic dipole moment M varies between 2×10^22 and 10×10^22 Am^2. We study this for solar minimum, medium and maximum conditions.

Figure 1 shows how the magnetopause stand-off distance R_s decreases and the shape of the magnetosphere changes as M decreases. While a decrease in R_s is expected from a theoretical balance between the solar wind dynamic pressure and the magnetic pressure inside the magnetosphere (indicated by the black line), the model shows a stronger dependence on M. This may be due to enhanced magnetopause erosion and/or to strong changes in the ionospheric conductance, which affect the field-aligned currents. The magnetic fields created by the field-aligned currents in the magnetosphere modify the magnetic pressure inside the magnetosphere, which changes the dependence of R_s on M as well as the shape of the magnetosphere, here indicated by the ratio of the distance to the magnetopause in the solar magnetic y- and x-directions.

Changes in M also affect the ionosphere-thermosphere system. E×B drift velocities, Joule heating power, the global mean thermospheric temperature and the global mean height of the peak of the ionospheric F_2 layer, h_mF_2, all increase with increasing M for low dipole moments, but all decrease with increasing M for larger dipole moments.The peak electron density of the F_2 layer, N_mF_2, shows the opposite behaviour. The Sq amplitude decreases with increasing M and this dependence can be roughly described by a power law scaling. Most scaling relations show a weak dependence on the solar activity level, which is likely associated with a change in the relative contributions to the Pedersen conductance from the upper and lower ionosphere, which scale differently with M.

Figure caption: The stand-off distance R_s (top) and the ratio between the stand-off to the flank and nose of the magnetosphere (bottom) as a function of dipole moment for solar minimum (blue), medium (green) and maximum (red) conditions. The black line indicates the theoretical relationship R_s α M^1/3.

(2) Ingrid Cnossen, Michael Wiltberger, and Jeremy E. Ouellette, 2012: The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind-magnetosphere-ionosphere coupling

Figure 2: High resolution

The angle µ between the geomagnetic dipole axis and the geocentric solar magnetospheric (GSM) z-axis, sometimes called the “dipole tilt”, varies as a function of UT and season. Observations have shown that the cross-polar cap potential tends to maximize near the equinoxes, when on average µ = 0, with smaller values observed near the solstices. This is similar to the well-known semi-annual variation in geomagnetic activity. We used numerical model simulations to investigate how seasonal and diurnal variations in m influence the magnetosphere-ionosphere system. We found that variations in solar wind-magnetosphere coupling, largely associated with variations in the magnetic reconnection rate, are responsible for 70–90% of variations in cross-polar cap potential with µ. This is illustrated in figure 2, which shows the mean strength of the electric field along the separator line for equinox and solstice for a 24-hour average. A larger electric field corresponds to a higher reconnection rate. At equinox the section of the separator line along which relatively strong reconnection occurs is much longer than at solstice, leading to a higher cross-polar cap potential. Variations in the high-latitude ionospheric conductance with m also contribute to seasonal and diurnal variations in cross-polar cap potential, but this accounts only for 10–30% of the variation. For other variables, such as field-aligned currents and geomagnetic activity, variations in ionospheric conductance may play a relatively larger role.

Figure 2 caption: The 24-hour mean position of the separator line for March equinox and June solstice for simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model for southward IMF, colour-coded with the mean strength of the electric field parallel to the separator line.

(3) Ingrid Cnossen, and Arthur D. Richmond, 2012: How changes in the tilt angle of the geomagnetic dipole affect the coupled magnetosphere-ionosphere-thermosphere system

Figure 3: High resolution

The orientation of the Earth's magnetic field has changed dramatically during the geological past. We investigated the effects of changes in dipole tilt angle on the magnetosphere, ionosphere and thermosphere, using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model.

A change in dipole tilt angle changes the inclination of the magnetic field in a geographic reference frame. Because charged particles move much more easily along magnetic field lines than across them, this affects the vertical component of plasma transport processes, which in turn alters the vertical plasma distribution. This leads to changes in the height of the peak of the F_2 layer, h_mF_2, and its peak electron density, N_mF_2. An example for tilt angles of 0° (T0, dipole axis aligned with the rotation axis) and a tilt angle of 30° (T30) and the difference between them is shown in figure 3.

About 2/3 of the changes in N_mF_2 shown in figure 1, and most of the low to mid-latitude changes in h_mF_2, are due to changes in the vertical component of plasma diffusion along the magnetic field. The remainder is associated with changes in the amount of Joule heating, arising from changes in the efficiency of solar wind-magnetosphere coupling, and its geographic distribution, as the locations of the magnetic poles and auroral ovals change. Joule heating effects are only important when Joule heating is relatively high, as under southward Interplanetary Magnetic Field (IMF). Under northward IMF they are negligible.

Figure 3 caption: H_mF_2 and N_mF_2 for 0° dipole tilt (T0) and 30° dipole tilt (T30) and the difference between them (T30–T0) for equinox under southward Interplanetary Magnetic Field (IMF) conditions.

FY2012 Published Papers

Cnossen, I., M. Wiltberger, and J.E. Ouellette, 2012: The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind-magnetosphere-ionosphere coupling, J. Geophys. Res., in press, doi: 10.1029/2012JA017825.

Cnossen, I., and A.D. Richmond, 2012: How changes in the tilt angle of the geomagnetic dipole affect the coupled magnetosphere-ionosphere-thermosphere system, J. Geophys. Res., 117, A10317, doi: 10.1029/2012JA018056.

Lu, H., D. Pancheva, P. Mukhtarov, and I. Cnossen, 2012: The vertical structure of the QBO-modulated planetary wave activity during northern winter, J. Geophys. Res., 117, D09104, doi: 10.1029/2011JD016901.

Cnossen, I., A.D. Richmond, and M. Wiltberger, 2012: The dependence of the coupled magnetosphere-ionosphere-thermosphere system on the Earth's magnetic dipole moment, J. Geophys. Res., 117, A05302, doi: 10.1029/2012JA017555.

Cnossen, I., 2012: Climate change in the upper atmosphere, in: Greenhouse Gases—Emission, Measurement and Management, edited by Liu, G., InTech, ISBN 978-953-51-0323-3, p. 315–336.