Ingrid Cnossen

The response of the coupled magnetosphere‐ionosphere‐thermosphere system to a 25% reduction in the dipole moment of the Earth's magnetic field

[1]xa0The Earths magnetic field changes in orientation and strength over time. We study the response of the magnetosphere-ionosphere-thermosphere system to a 25% reduction in magnetic field intensity, using the coupled magnetosphere-ionosphere-thermosphere (CMIT) model. Simulations were performed with a dipole moment of 8 × 1022 A m2, close to the present-day value, and a dipole moment of 6 × 1022 A m2, both under the same solar wind conditions, intermediate solar activity (F10.7 = 150), and for March equinox and June solstice. The 25% reduction in field strength causes the magnetosphere to shrink and the polar cap to expand, in agreement with theory. The Pedersen and the Hall ionospheric conductances increase by 50%–60% and 60%–65%, respectively. This causes a ∼9%–12% decrease in electric potential and a ∼20% increase in field-aligned currents during equinox. Ion E × B drift velocities are enhanced by ∼10%–15%. The Joule heating also increases, by 13%–30%, depending on the season. Changes in the neutral temperature structure are caused partly by changes in Joule heating and partly by changes in the neutral wind. The neutral wind itself is also affected by changes in neutral temperature and by changes in ion velocities. The changes in the neutral wind, together with changes in the vertical component of the E × B drift, affect the height of the ionospheric F2 layer. Changes in electron density are related to changes in the O/N2 ratio. The global mean increase in neutral temperature causes the thermosphere to expand, resulting in a global mean uplift of the ionosphere. These effects are generally smaller during solstice.

How changes in the tilt angle of the geomagnetic dipole affect the coupled magnetosphere-ionosphere-thermosphere system

[1]xa0The orientation of the Earths magnetic field has changed dramatically during the geological past. We have investigated the effects of changes in dipole tilt angle on the magnetosphere, ionosphere, and thermosphere, using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model. The dipole tilt angle modulates the efficiency of solar wind-magnetosphere coupling, by influencing the diurnal variation in the angleμ between the dipole axis and the GSM z axis. This influences how much Joule heating occurs at high magnetic latitudes. The dipole tilt angle also controls the geographic distribution of the Joule heating, as it determines the geographic latitude of the magnetic poles. Changes in the amount and distribution of Joule heating with tilt an`gle produce further changes in temperature and neutral winds. The latter affect the O/N2 ratio, which in turn modifies the peak electron density of the F2 layer, NmF2. All these effects are most important when the Interplanetary Magnetic Field (IMF) is southward, while being almost negligible under northward IMF. However, a change in dipole tilt also changes the inclination of the magnetic field, which affects the vertical component of ionospheric plasma diffusion along the magnetic field, regardless of the IMF direction. Changes in vertical plasma diffusion are responsible for ∼2/3 of the changes in NmF2 and most of the low to midlatitude changes in hmF2under southward IMF and for most of the changes in both variables under northward IMF. Thermal contraction may be responsible for high-latitude decreases in hmF2 with increasing tilt angle under southward IMF.

The dependence of the coupled magnetosphere-ionosphere- thermosphere system on the Earth's magnetic dipole moment

[1] 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 find that the magnetopause stand-off distance and the cross-polar cap potential increase, while the polar cap size decreases, with increasing M. Their dependence on M is stronger than predicted by previous studies. We also show for the first time that the shape of the magnetosphere starts to change for M ≤ 4⋅10 22 Am 2 . This may be due to enhanced magnetopause erosion and/or to strong changes in the ionospheric conductance, which affect the field-aligned currents and the magnetic fields they create in the magnetosphere,

Changes in the Earth's magnetic field over the past century: Effects on the ionosphere‐thermosphere system and solar quiet (Sq) magnetic variation

[1]xa0We investigated the contribution of changes in the Earths magnetic field to long-term trends in the ionosphere, thermosphere, and solar quiet (Sq) magnetic variation using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model. Simulations with the magnetic fields of 1908, 1958, and 2008 were done. The strongest differences occurred between ~40°S–40°N and ~100°W–50°E, which we refer to as the Atlantic region. The height and critical frequency of the F2 layer peak, hmF2 and foF2, changed due to changes in the vertical Eu2009×u2009B drift and the vertical components of diffusion and transport by neutral winds along the magnetic field. Changes in electron density resulted in changes in electron temperature of the opposite sign, which in turn produced small corresponding changes in ion temperature. Changes in neutral temperature were not statistically significant. Strong changes in the daily amplitude of the Sq variation occurred at low magnetic latitudes due to the northward movement of the magnetic equator and the westward drift of the magnetic field. The simulated changes in hmF2, foF2, and Sq amplitude translate into typical trends of ±1u2009km/decade (night) to ±3u2009km/decade (day), −0.1 to +0.05u2009MHz/decade, and ±5 to ±10u2009nT/century, respectively. These are mostly comparable in magnitude to observed trends in the Atlantic region. The simulated Atlantic region trends in hmF2 and foF2 are ~2.5 times larger than the estimated effect of enhanced greenhouse gases on hmF2 and foF2. The secular variation of the Earths magnetic field may therefore be the dominant cause of trends in the Atlantic region ionosphere.

The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere‐ionosphere coupling

Received 13 April 2012; revised 19 September 2012; accepted 10 October 2012; published 16 November 2012. [1] The angle m 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 m = 0, with smaller values observed near the solstices. This is similar to the well-known semiannual variation in geomagnetic activity. We use numerical model simulations to investigate the role of two possible mechanisms that may be responsible for the influence of m on the magnetosphere-ionosphere system: variations in the coupling efficiency between the solar wind and the magnetosphere and variations in the ionospheric conductance over the polar caps. Under southward interplanetary magnetic field (IMF) conditions, variations in ionospheric conductance at high magnetic latitudes are responsible for 10–30% of the variations in the cross-polar cap potential associated with m, but variations in solar wind–magnetosphere coupling are more important and responsible for 70–90%. Variations in viscous processes contribute slightly to this, but variations in the reconnection rate with m are the dominant cause. The variation in the reconnection rate is primarily the result of a variation in the length of the section of the separator line along which relatively strong reconnection occurs. Changes in solar wind–magnetosphere coupling also affect the field-aligned currents, but these are influenced as well by variations in the conductance associated with variations in m, more so than the cross-polar cap potential. This may be the case for geomagnetic activity too.

The role of the Sun in long‐term change in the F2 peak ionosphere: New insights from EEMD and numerical modeling

We applied Ensemble Empirical Mode Decomposition (EEMD) for the first time to ionosonde data to study trends in the critical frequency of the F2 peak, foF2, and its height, hmF2, from 1959 to 2005. EEMD decomposes a time series into several quasi-cyclical components, called Intrinsic Mode Functions (IMFs), and a residual, which can be interpreted as a long-term trend. In contrast to the more commonly used linear regression-based trend analysis, EEMD makes no assumptions on the functional form of the trend and no separate correction for the influence of solar activity variations is needed. We also adopted a more rigorous significance testing procedure with less restrictive underlying assumptions than the F-test, which is normally used as part of a linear regression-based trend analysis. EEMD analysis shows that trends in hmF2 and foF2 between 1959 and 2005 are mostly highly linear, but the F-test tends to overestimate the significance of trends in hmF2 and foF2 in 30% and 25% of cases, respectively. EEMD-based trends are consistently more negative than linear regression-based trends, by 30-35% for hmF2 and about 50% for foF2. This may be due to the different treatment of the influence of a long-term decrease in solar activity from 1959 to 2005. We estimate the effect of this decrease in solar activity with two different data-based methods as well as using numerical model simulations. While these estimates vary, all three methods demonstrate a larger relative influence of the Sun on trends in foF2 than on trends in hmF2.

North‐south asymmetries in the polar thermosphere‐ionosphere system: Solar cycle and seasonal influences

Previous studies have revealed that ion drift and neutral wind speeds at ~400u2009km in the polar cap (>80° magnetic latitude) are on average larger in the Northern Hemisphere (NH) than in the Southern Hemisphere, which is at least partly due to asymmetry in the geomagnetic field. Here we investigate for the first time how these asymmetries depend on season and on solar/geomagnetic activity levels. Ion drift measurements from the Cluster mission show little seasonal dependence in their north-south asymmetry when all data (February 2001–December 2013) are used, but the asymmetry disappears around June solstice for high solar activity and around December solstice for low solar activity. Neutral wind speeds in the polar cap obtained from the Challenging Minisatellite Payload spacecraft (January 2002–December 2008) are always larger in the summer hemisphere, regardless of solar activity, but the high-latitude neutral wind vortices at dawn and dusk tend to be stronger in the NH, except around December solstice, in particular, when solar activity is low. Simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) more or less capture the behavior of the ion drift speeds, which can be explained as a superposition of seasonal and geomagnetic field effects, with the former being stronger for higher solar activity. The behavior of the neutral wind speed and vorticity is not accurately captured by the model. This is probably due to an incorrect seasonal cycle in plasma density around ~400u2009km in CMIT, which affects the ion drag force. This must be addressed in future work.

Solar signal propagation: The role of gravity waves and stratospheric sudden warmings

We use a troposphere-stratosphere model of intermediate complexity to study the atmospheric response to an idealized solar forcing in the subtropical upper stratosphere during Northern Hemisphere (NH) early winter. We investigate two conditions that could influence poleward and downward propagation of the response: (1) the representation of gravity wave effects and (2) the presence/absence of stratospheric sudden warmings (SSWs). We also investigate how the perturbation influences the timing and frequency of SSWs. Differences in the poleward and downward propagation of the response within the stratosphere are found depending on whether Rayleigh friction (RF) or a gravity wave scheme (GWS) is used to represent gravity wave effects. These differences are likely related to differences in planetary wave activity in the GWS and RF versions, as planetary wave redistribution plays an important role in the downward and poleward propagation of stratospheric signals. There is also remarkable sensitivity in the tropospheric response to the representation of the gravity wave effects. It is most realistic for GWS. Further, tropospheric responses are systematically different dependent on the absence/presence of SSWs. When only years with SSWs are examined, the tropospheric signal appears to have descended from the stratosphere, while the signal in the troposphere appears disconnected from the stratosphere when years with SSWs are excluded. Different troposphere-stratosphere coupling mechanisms therefore appear to be dominant for years with and without SSWs. The forcing does not affect the timing of SSWs, but does result in a higher occurrence frequency throughout NH winter. Quasi-Biennial Oscillation effects were not included.

North–South Asymmetries in Earth’s Magnetic Field. Effects on High-Latitude Geospace

The solar-wind magnetosphere interaction primarily occurs at altitudes where the dipole component of Earth’s magnetic field is dominating. The disturbances that are created in this interaction propagate along magnetic field lines and interact with the ionosphere–thermosphere system. At ionospheric altitudes, the Earth’s field deviates significantly from a dipole. North–South asymmetries in the magnetic field imply that the magnetosphere–ionosphere–thermosphere (M–I–T) coupling is different in the two hemispheres. In this paper we review the primary differences in the magnetic field at polar latitudes, and the consequences that these have for the M–I–T coupling. We focus on two interhemispheric differences which are thought to have the strongest effects: 1)xa0Axa0difference in the offset between magnetic and geographic poles in the Northern and Southern Hemispheres, and 2)xa0differences in the magnetic field strength at magnetically conjugate regions. These asymmetries lead to differences in plasma convection, neutral winds, total electron content, ion outflow, ionospheric currents and auroral precipitation.

The vertical connection of the quasi‐biennial oscillation‐modulated 11 year solar cycle signature in geopotential height and planetary waves during Northern Hemisphere early winter

[1] We analyzed observational geopotential height data to provide some new insights on the 11 year solar cycle signal in the Northern Hemisphere early winter and its modulation by the quasi‐biennial oscillation (QBO). The signals are strongest in the upper stratosphere. When the QBO is in its easterly phase (QBOe), it appears to move gradually eastward and poleward, resulting in a predominantly positive signal over the pole, with a weaker vertically connected negative signal over the Icelandic Low. When the QBO is in its westerly phase (QBOw), the polar stratospheric signal is mainly negative and appears connected to a negative anomaly in the troposphere over the Aleutian Low. A spectral analysis of the stratospheric response in planetary waves showed a reduction of wave number 2 power under QBOe and an enhancement of wave number 3 under QBOw. These responsesarecharacterizedbyanoverallincrease/decreaseinwaveactivityatmiddletohigh latitudes rather than a latitudinal shift of wave activity. There is no clear stratosphere‐ troposphere connection under QBOe, but under QBOw, there is a vertically coherent increase in wave power at wave numbers 1–3 with a period of 5.6–6.9 days. We suggest that the differences in response under QBOe and QBOw can be explained through differences in initial vortex strength, resulting in either a stronger influence from the low‐latitude upper stratosphere (QBOe) or from the troposphere (QBOw) on the polar stratosphere.