| 21. |
- Buchert, Stephan, et al.
(author)
-
SWARM observations of equatorial electron densities and topside GPS track losses
- 2015
-
In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:7, s. 2088-2092
-
Journal article (peer-reviewed)abstract
- The SWARM satellites have both upward looking GPS receivers and Langmuir probes. The receivers repeatedly lost track of the L1 band signal in January-February 2014 at postsunset hours, when SWARM was at nearly 500km altitude. This indicates that the signal was disturbed by ionospheric irregularities at this height and above. The track losses occurred right at density gradients associated with equatorial plasma bubbles and predominantly where the measured background density was highest. The signal showed strong phase scintillations rather than in amplitude, indicating that SWARM might be in the near field of an ionospheric phase screen. Density biteouts, depletions between steep gradients, were up to almost 3 orders of magnitude deep in the background of a more shallow trough centered at the magnetic equator. Comparison between satellites shows that the biteout structure strongly varied in longitude over approximate to 100km and has in north-south steep walls.
|
|
| 22. |
- Bulatovic, Ines, et al.
(author)
-
Aerosol Indirect Effects in Marine Stratocumulus : The Importance of Explicitly Predicting Cloud Droplet Activation
- 2019
-
In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:6, s. 3473-3481
-
Journal article (peer-reviewed)abstract
- Climate models generally simulate a unidirectional, positive liquid water path (LWP) response to increasing aerosol number concentration. However, satellite observations and large-eddy simulations show that the LWP may either increase or decrease with increasing aerosol concentration, influencing the overall magnitude of the aerosol indirect effect (AIE). We use large-eddy simulation to investigate the LWP response of a marine stratocumulus cloud and its dependence on different parameterizations for obtaining cloud droplet number concentration (CDNC). The simulations confirm that the LWP response is not always positiveregardless of CDNC treatment. However, the AIE simulated with the model version with prescribed CDNC is almost 3 times larger compared to the version with prognostic CDNC. The reason is that the CDNC in the prognostic scheme varies in time due to supersaturation fluctuations, collection, and other microphysical processes. A substantial spread in simulated AIE may thus arise simply due to the CDNC treatment. Plain Language Summary Our poor understanding of aerosol-cloud-radiation interactions (aerosol indirect effects) results in a major uncertainty in estimates of anthropogenic aerosol forcing. In climate models, the cloud water response to an increased aerosol number concentration may be especially uncertain as models simplify, or do not account for, processes that affect the cloud droplet number concentration and the total amount of cloud water. In this study, we employ large-eddy simulation to explore how different model descriptions for obtaining the number concentration of cloud droplets influences the cloud water response of a marine stratocumulus cloud and thus the simulated aerosol indirect effect. Our simulations show a qualitatively similar cloud water response regardless of model description: the total amount of cloud water increases first and then decreases with increasing aerosol concentration. However, the simulated aerosol indirect effect is almost 3 times as large when the number concentration of cloud droplets is prescribed compared to when it is dependent on the calculated supersaturation and other microphysical processes such as collisions between cloud droplets. Our findings show that a relatively simple difference in the treatment of the number concentration of cloud droplets in climate models may result in a significant spread in the simulated aerosol indirect effect.
|
|
| 23. |
- Burch, J. L., et al.
(author)
-
High-Frequency Wave Generation in Magnetotail Reconnection : Linear Dispersion Analysis
- 2019
-
In: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 46:8, s. 4089-4097
-
Journal article (peer-reviewed)abstract
- Plasma and wave measurements from the NASA Magnetospheric Multiscale mission are presented for magnetotail reconnection events on 3 July and 11 July 2017. Linear dispersion analyses were performed using distribution functions comprising up to six drifting bi-Maxwellian distributions. In both events electron crescent-shaped distributions are shown to be responsible for upper hybrid waves near the X-line. In an adjacent location within the 3 July event a monodirectional field-aligned electron beam drove parallel-propagating beam-mode waves. In the 11 July event an electron distribution consisting of a drifting core and two crescents was shown to generate upper-hybrid and beam-mode waves at three different frequencies, explaining the observed broadband waves. Multiple harmonics of the upper hybrid waves were observed but cannot be explained by the linear dispersion analysis since they result from nonlinear beam interactions. Plain Language Summary Magnetic reconnection is a process that occurs throughout the universe in ionized gases (plasmas) containing embedded magnetic fields. This process converts magnetic energy to electron and ion energy, causing phenomena such as solar flares and auroras. The NASA Magnetospheric Multiscale mission has shown that in magnetic reconnection regions there are intense electric field oscillations or waves and that electrons form crescent and beam-like populations propagating both along and perpendicular to the magnetic field. This study shows that the observed electron populations are responsible for high-frequency waves including their propagation directions and frequency ranges.
|
|
| 24. |
- Burch, J. L., et al.
(author)
-
Localized Oscillatory Energy Conversion in Magnetopause Reconnection
- 2018
-
In: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 45:3, s. 1237-1245
-
Journal article (peer-reviewed)abstract
- Data from the NASA Magnetospheric Multiscale mission are used to investigate asymmetric magnetic reconnection at the dayside boundary between the Earth's magnetosphere and the solar wind. High-resolution measurements of plasmas and fields are used to identify highly localized (similar to 15 electron Debye lengths) standing wave structures with large electric field amplitudes (up to 100 mV/m). These wave structures are associated with spatially oscillatory energy conversion, which appears as alternatingly positive and negative values of J . E. For small guide magnetic fields the wave structures occur in the electron stagnation region at the magnetosphere edge of the electron diffusion region. For larger guide fields the structures also occur near the reconnection X-line. This difference is explained in terms of channels for the out-of-plane current (agyrotropic electrons at the stagnation point and guide field-aligned electrons at the X-line).
|
|
| 25. |
- Cael, B. B., et al.
(author)
-
The volume and mean depth of Earth's lakes
- 2017
-
In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 44:1, s. 209-218
-
Journal article (peer-reviewed)abstract
- Global lake volume estimates are scarce, highly variable, and poorly documented. We developed a rigorous method for estimating global lake depth and volume based on the Hurst coefficient of Earth's surface, which provides a mechanistic connection between lake area and volume. Volume-area scaling based on the Hurst coefficient is accurate and consistent when applied to lake data sets spanning diverse regions. We applied these relationships to a global lake area census to estimate global lake volume and depth. The volume of Earth's lakes is 199,000km(3) (95% confidence interval 196,000-202,000km(3)). This volume is in the range of historical estimates (166,000-280,000km(3)), but the overall mean depth of 41.8m (95% CI 41.2-42.4m) is significantly lower than previous estimates (62-151m). These results highlight and constrain the relative scarcity of lake waters in the hydrosphere and have implications for the role of lakes in global biogeochemical cycles.
|
|
| 26. |
- Cao, D., et al.
(author)
-
MMS observations of whistler waves in electron diffusion region
- 2017
-
In: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 44:9, s. 3954-3962
-
Journal article (peer-reviewed)abstract
- Whistler waves that can produce anomalous resistivity by affecting electrons' motion have been suggested as one of the mechanisms responsible for magnetic reconnection in the electron diffusion region (EDR). Such type of waves, however, has rarely been observed inside the EDR so far. In this study, we report such an observation by Magnetospheric Multiscale (MMS) mission. We find large-amplitude whistler waves propagating away from the X line with a very small wave-normal angle. These waves are probably generated by the perpendicular temperature anisotropy of the -300eV electrons inside the EDR, according to our analysis of dispersion relation and cyclotron resonance condition; they significantly affect the electron-scale dynamics of magnetic reconnection and thus support previous simulations.
|
|
| 27. |
- Cattell, C., et al.
(author)
-
Dayside response of the magnetosphere to a small shock compression : Van Allen Probes, Magnetospheric MultiScale, and GOES-13
- 2017
-
In: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 44:17, s. 8712-8720
-
Journal article (peer-reviewed)abstract
- Observations from Magnetospheric MultiScale (similar to 8 Re) and Van Allen Probes (similar to 5 and 4 Re) show that the initial dayside response to a small interplanetary shock is a double-peaked dawnward electric field, which is distinctly different from the usual bipolar (dawnward and then duskward) signature reported for large shocks. The associated ExB flow is radially inward. The shock compressed the magnetopause to inside 8 Re, as observed by Magnetospheric MultiScale (MMS), with a speed that is comparable to the ExB flow. The magnetopause speed and the ExB speeds were significantly less than the propagation speed of the pulse from MMS to the Van Allen Probes and GOES-13, which is consistent with the MHD fast mode. There were increased fluxes of energetic electrons up to several MeV. Signatures of drift echoes and response to ULF waves also were seen. These observations demonstrate that even very weak shocks can have significant impact on the radiation belts. Plain Language Summary Very fast moving clouds of charged particles are ejected from the Sun when it is active. Shock waves often develop at the cloud front as it plows through the solar wind. When the shock hits the Earth's magnetic field, it can push the Earth's magnetic shield inside the distance where many communication and weather satellites orbit. The energy associated with the shock can also very rapidly increase the energy of electrons trapped in the Earth's magnetic field in the Van Allen Radiation belts. These electrons can damage satellites. We have used four satellites arrayed at different locations on the dayside of the Earth's magnetic field to show, for the first time, that small shocks have a different effect than the large shocks that are usually studied but that even small shocks can produce relativistic electrons.
|
|
| 28. |
- Cazzola, E., et al.
(author)
-
On the electron agyrotropy during rapid asymmetric magnetic island coalescence in presence of a guide field
- 2016
-
In: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 43:15, s. 7840-7849
-
Journal article (peer-reviewed)abstract
- We present an analysis of the properties of the electron velocity distribution during island coalescence in asymmetric reconnection with and without guide field. In a previous study, three main domains were identified, in the case without guide field, as X, D, and M regions featuring different reconnection evolutions. These regions are also identified here in the case with guide field. We study the departure from isotropic and gyrotropic behavior by means of different robust detection algorithms proposed in the literature. While in the case without guide field these metrics show an overall agreement, when the guide field is present, a discrepancy in the agyrotropy within some relevant regions is observed, such as at the separatrices and inside magnetic islands. Moreover, in light of the new observations from the Multiscale MagnetoSpheric mission, an analysis of the electron velocity phase-space in these domains is presented.
|
|
| 29. |
- Chafik, L., et al.
(author)
-
Global linkages originating from decadal oceanic variability in the subpolar North Atlantic
- 2016
-
In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 43:20, s. 10909-10919
-
Journal article (peer-reviewed)abstract
- The anomalous decadal warming of the subpolar North Atlantic Ocean (SPNA), and the northward spreading of this warm water, has been linked to rapid Arctic sea ice loss and more frequent cold European winters. Recently, variations in this heat transport have also been reported to covary with global warming slowdown/acceleration periods via a Pacific climate response. We here examine the role of SPNA temperature variability in this Atlantic-Pacific climate connectivity. We find that the evolution of ocean heat content anomalies from the subtropics to the subpolar region, likely due to ocean circulation changes, coincides with a basin-wide Atlantic warming/cooling. This induces an Atlantic-Pacific sea surface temperature seesaw, which in turn, strengthens/weakens the Walker circulation and amplifies the Pacific decadal variability that triggers pronounced global-scale atmospheric circulation anomalies. We conclude that the decadal oceanic variability in the SPNA is an essential component of the tropical interactions between the Atlantic and Pacific Oceans.
|
|
| 30. |
- Chafik, Léon, et al.
(author)
-
Volume, Heat, and Freshwater Divergences in the Subpolar North Atlantic Suggest the Nordic Seas as Key to the State of the Meridional Overturning Circulation
- 2019
-
In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:9, s. 4799-4808
-
Journal article (peer-reviewed)abstract
- The meridional overturning circulation (MOC) decreases rapidly in subpolar and Nordic regions where the warm upper layer loses its buoyancy due to intense heat loss, sinks, and flows south. The major volume loss of the upper limb of the MOC, similar to 9.6 Sv out of 18.4 +/- 3.4 Sv, occurs as subduction across the Iceland Basin and Irminger Sea while the major heat loss, 273 TW out of 395 +/- 74 TW is associated with the MOC branch that continues into the Nordic Seas where North Atlantic deep overflow water is produced. The 122 +/- 79 TW heat flux convergence in the subpolar gyre appears to be significantly larger than various estimates of heat loss to the atmosphere. Much of the 0.09 +/- 0.02 Sv freshwater divergence is presumably balanced by runoff from the Greenland shelf. These estimates suggest that the Nordic Seas, not the Labrador Sea, are key to the state of the MOC. Plain language summary The meridional overturning circulation is a two-dimensional view of the flow north of upper-ocean warm water and its return south as cold deep and intermediate water. But the actual pathways of warm-to-cold conversion are several and remarkably diverse: One branch continues into the Nordic Seas where very dense water is produced and eventually spills back into the deep North Atlantic, another branch weaves its way around the entire subpolar basin and the southern tip of Greenland to the Labrador Sea where intermediate water is formed, and the third branch is an overturning that takes place within the subpolar waters between Greenland and Scotland. Volumetrically, this is the largest branch, but in terms of heat loss the Nordic Seas, branch surrenders far more heat to the atmosphere than the other two combined. It thus plays the key role in maintaining a strong meridional overturning circulation.
|
|
