ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-35-879-2017Investigation of ∼20–40 mHz ULF waves and their driving mechanisms in Mercury's dayside magnetosphereLiljebladElisabetelilil@kth.sehttps://orcid.org/0000-0002-9164-0761KarlssonTomasDepartment of Space and Plasma Physics, School of Electrical Engineering, Royal Institute of Technology (KTH), Stockholm, SwedenElisabet Liljeblad (elilil@kth.se)28July201735487988428March201730May20175July2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/35/879/2017/angeo-35-879-2017.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/35/879/2017/angeo-35-879-2017.pdf
Ultra-low-frequency (ULF) waves in the ∼ 20–40 mHz range are
frequently observed in the Mercury magnetosphere using Mercury Surface Space
Environment Geochemistry, and Ranging (MESSENGER) magnetic field data. The
majority of these waves have very similar characteristics to the waves likely
driven by Kelvin–Helmholtz (KH) ULF waves (which are retained as a subset of
the wave events studied in this paper) identified in a previous study.
Significant ULF wave activity is observed in the dawn sector of the
magnetosphere. This indicates that Mercury KH waves may be more common
between 6 and 12 magnetic local time than previously predicted and that
magnetospheric ULF waves in the frequency band ∼20–40 mHz can be used
as a detection tool for Hermean KH waves.
Ultra-low-frequency (ULF) waves were first observed in the magnetic
environment of Mercury with Mariner 10 . These were
narrowband ∼0.5 Hz waves with a polarization changing from right-hand
circular to linear as the spacecraft moved towards the planet. Since the
arrival of Mercury Surface Space Environment Geochemistry, and Ranging
(MESSENGER), Mercury magnetospheric ULF waves have been observed a number
of times, ranging from ∼ mHz up to
∼1 Hz .
In the ULF wave context, much focus has been directed toward the nature of
the ULF wave generation. introduced the idea that certain
ULF waves detected in the terrestrial magnetosphere could be standing
Alfvén waves on geomagnetic field lines referred to as field line
resonances (FLRs). Different internal and external driving mechanisms, such as resonance with energetic particle populations
e.g.,, pressure fluctuations in the solar wind
e.g., or the Kelvin–Helmholtz instability (KHI)
e.g.,, have
been proposed. In particular, terrestrial toroidal-mode Pc 5
pulsations have been proposed to likely be driven by the KHI
e.g.,.
The study by investigated all 131 MESSENGER
magnetospheric traversals just prior to or after the observation of
Kelvin–Helmholtz waves (KHWs) at the magnetopause during 2011–2013, which in
turn were identified in an earlier study by . Distinct
ULF wave signatures in the mHz range could be detected in 44 out of these 131
magnetospheric passages. The KHWs situated at the dayside could be connected
to clear ULF wave activity nearly 50 % of the time. The ULF waves followed
the KHW occurrence asymmetry and appear mainly at the duskside magnetopause
between 14 and 17 magnetic local time. These waves were observed most often in
the narrow frequency range of 20–40 mHz and in the same range as the KHWs.
The overall results, including similar characteristics and the close temporal
connection between the ULF waves and the KHWs, argue for the KHI as a driver.
These results manifest the importance of the KHI for the energy and momentum
transport throughout the Mercury magnetic system and motivate a general
study of magnetospheric ULF waves, in particular in the 20–40 mHz frequency
range, to learn more about possible KHI-driven ULF waves.
This study will analyze the magnetic field of all dayside magnetospheric
crossings of MESSENGER during the year 2011 to identify clear ULF wave activity.
Such ULF waves will be investigated by evaluation of their characteristics
and by comparing them to the dayside likely KHI-generated ULF waves reported
in to discern possible driving mechanisms.
Observations and results
During 2011, MESSENGER crossed the dayside magnetosphere 542 times. Eight of
these occurred in direct connection to a KHW and have thus already been
investigated for ULF wave activity . In the present
study, the remaining 534 traversals are investigated. Using the procedure and
criteria set by , in which a 2.5 s running average and a
subtraction of a 50 s running mean value is applied to make any
quasi-periodic signature between 0.02 and 0.4 Hz more clearly visible, and only
events with a power spectral density of at least 1000 nT2 Hz-1 are
included, 60 clear ULF wave events are identified, each on a separate
traversal. An example of a magnetospheric traversal with such ULF wave
activity can be seen in Fig. .
The upper panel shows an example of a magnetospheric traversal,
in which red lines mark the ULF wave signature and the blue line marks the
magnetopause. The lower panel shows the detrended and smoothed ULF wave
region.
The methods by and , described in
, are used for the derivation of polarization parameters
such as coherence; ellipticity, ϵ; and wave normal angle, α.
These parameters are derived only for those 39 events fulfilling the criteria
of coherence >70 % and eigenvalue ratio (intermediate to minimum)
λint/λmin>4 of the eigenvectors of the
spectral matrix. The data set including these 39 ULF events is analyzed and
will from hereon be referred to as general ULF waves.
As mentioned above, the events from were identified in
direct connection to a KHI at the magnetopause. Of these events, 43 were
detected in the dayside magnetosphere and 38 of these fulfill the same
criteria as outlined above. The latter 38 have been included in this study as
a reference and will from hereon be referred to as KHI-ULF waves.
Results of the analysis of the total 77 events from both the general and
KHI-ULF wave group can be seen in Fig. . The large
majority of both general ULF waves (circles) and KHI-ULF waves (triangles)
clearly have a large perpendicular power spectral density component and a
larger azimuthal than radial component, indicative of a shear Alfvénic
wave type. On the dawnside, the majority of ULF events in both data sets are
left-hand polarized (ϵ<0) with respect to the background magnetic
field, while the duskside events are mainly right-hand polarized. Another
distinct result is that the total power spectral density is generally larger
for the KHI-ULF events (on average 6870±760 nT2 Hz-1,
including standard error) than the general ULF events (on average 3850±1090 nT2 Hz-1), which is possibly explained by their distance in
time to a KHI (see Sect. ). The fact that the majority of
events in both data sets are in the narrow frequency range of 20–40 mHz
indicates that they might be driven by the same mechanism. Adding to this
idea is the result that the majority of both general ULF waves and KHI-ULF
waves, both at dusk and dawn, propagate more in the parallel than in the
perpendicular direction; see Fig. f. The events with
perpendicular to total power spectral density less than 0.7 (marked by
empty symbols) have clearly different characteristics than the other ones,
suggesting either that they are a different type of KHI-driven wave (e.g.,
compressional waves in an early stage before entering a region of an FLR
where the waves are more azimuthally polarized) or that they are driven by a
different mechanism.
Results versus magnetic local time (MLT) of ULF events fulfilling
the polarization analysis criteria outlined in Sect. .
(a) Perpendicular over total power spectral density,
(b) azimuthal over radial power spectral density in log scale,
(c) absolute ellipticity, (d) total power spectral density
in log scale, (e) frequency, and (f) wave normal angle, all
evaluated at maximum power spectral density. Red and black colors represent
positive and negative ellipticity, respectively; dots and triangles represent
general ULF events and KHI-ULF events, respectively; and empty symbols
represent events with a low perpendicular over total power spectral density
(<0.7).
The obvious similarity between the duskside events of the general and KHI-ULF
wave group is more clearly visible in Fig. .
Again, the majority of events of both data sets are positively polarized with
a significant perpendicular component, they decrease in azimuthal power with
distance from noon, and they are in the same frequency range with similar wave
normal angles.
Comparison of the two data sets' general ULF waves and KHI-ULF waves,
in which only duskside events are included. Panels and symbols follow the same
format as in Fig. .
The location of the events, which can be seen in
Fig. , shows that the KHI-ULF waves (marked by red)
span over a larger region than the general ULF waves. Also significant is the
fact that a large number of the general ULF waves are observed at the dawnside, unlike the KHWs (and the KHI-ULF waves), which are observed mainly at
the duskside magnetopause (magnetosphere).
Location of ULF events in three different planes in Mercury solar
magnetospheric (MSM) coordinates, in which blue lines mark the general ULF
waves and red lines mark the KHI-ULF waves.
Discussion
The study by indicates that the identified ULF waves
are generated by KHWs at the magnetopause. In addition, a study by
covering MESSENGER magnetosphere crossings over the time
period 2011–2015 also suggests that ULF waves (occurring mainly at the
dayside), with similar characteristics as those KHI-ULF waves reported in
, could be KHI driven.
This study shows that the duskside general ULF waves are clearly similar to
the duskside KHI-ULF waves in all investigated characteristics. Due to a
small number of dawnside KHI-ULF events, it is not meaningful to compare
these to the dawnside general ULF waves. However, as can be seen, the
dawnside general ULF waves are similar to the duskside KHI-ULF waves with
regard to their relative perpendicular power spectral density, main frequency
and wave normal angle. As for the KHI-ULF events, they also have a larger
azimuthal than radial component but do not show a gradual change with
distance from noon in both the ratio of azimuthal to radial power spectral
density and absolute ellipticity; see Fig. b and c. In
addition, the dawnside general ULF waves most often have a negative
ellipticity in contrast to the positive ellipticity for the duskside general
ULF waves. However, this last discrepancy may not mean that the dusk- and
dawnside general ULF waves are driven by two different mechanisms. According
to and , and as visualized by
, the polarization of the KHI-driven ULF waves should be
different depending on if they are situated in the morning- or eveningside of
the magnetosphere. The result of opposite polarization in the dusk- and
dawnside magnetosphere thus supports the idea that the majority of not only
the duskside but also the dawnside general ULF waves are driven by the KHI.
Furthermore, observed the average polarization of their
observed ULF waves to be left-handed on the dawnside and right-handed on the
duskside (in agreement with the result in this study) and suggested this to
be a consequence of the KHI.
The total power spectral density is on average larger for the KHI-ULF waves
than the general ULF waves. Assuming that the majority of both general ULF
waves and KHI-ULF waves are KHI driven, this difference could simply be due
to their distance in time to a KHW. If the KHI-ULF waves are directly driven
by the KHW observed just ∼10 min earlier or after, the KHI-ULF waves
might not have time to decrease in amplitude much, resulting in a strong
magnetic signature and power spectral density for these ULF waves. The
general ULF waves could also be driven by KHWs, and if so they are probably
driven by weaker KHWs or are observed in a longer distance in time from
these, resulting in weaker magnetic ULF wave signatures. This argument can
also be used to explain why the KHI-ULF events are observed in a larger
region of the magnetosphere than the general ULF waves.
Also supporting the idea that the general ULF waves both at dusk and dawn are
KHI driven, is that their frequencies are mainly in the narrow band of
20–40 mHz, their wave normal angles are similar to those of the KHI-ULF
waves and the majority of events in both data sets are distinctly
azimuthally polarized. However, a few events clearly show different
characteristics (i.e., those marked by empty symbols) both in power spectral
density and wave normal angle. Hence, these could be driven by a different
mechanism and/or belong to a different type of ULF wave. As discussed in
, these waves might be compressional waves directly
driven by the KHI (before they enter into a region of an FLR where they
couple with the shear mode) or possibly cavity modes set up for example by
pressure fluctuations in the solar wind .
The large majority of KHWs observed in were identified
at the duskside magnetopause. As shown by , MESSENGER
covers the Hermean magnetosphere almost symmetrically during the year 2011.
Therefore, observing a large part of the general ULF waves at the dawnside
magnetosphere is surprising, assuming they are (as the duskside general ULF
waves) likely driven by the KHI. This either means that the dawnside general
ULF waves are not generated by the KHI (although our results indicate that
they are) or that dawnside KHWs are more common on Mercury than previously
thought. Perhaps these dawnside KHWs do indeed frequently develop at the
magnetopause but are constantly repressed by certain conditions of the
surrounding environment (such as a broader velocity shear layer or a
low-latitude boundary layer present most often at the dawnside as reported in
) and will therefore not be as clearly visible as the
duskside KHWs. Thus, they are more difficult to identify with the criteria used in
.
Conclusions
ULF waves in the ∼20–40 mHz range are frequently observed in the
Mercury magnetosphere. The majority of these (in particular those on the dusk
side) have very similar characteristics to the likely KHI-driven ULF waves
identified in a previous study . That a large number of
the ULF waves are observed in the dawn sector of the magnetosphere and show
similar characteristics as the KHI-driven ULF waves, including an opposite
polarization compared to the duskside general ULF waves, indicates that KHWs
at the dawnside may be more common than previously predicted. Assuming that
the KHI-ULF waves are indeed KHI driven, this study
advocates using ULF waves in
the ∼20–40 mHz frequency range to identify KHI activity at the
magnetopause. Thus, together with direct observations of KHWs at the
magnetopause, an estimation of how frequently the KHI occurs in the Hermean
magnetosphere can be obtained.
The data for this paper are available at
the NASA Planetary Data System: planetary plasma interactions node archive
(http://pds-ppi.igpp.ucla.edu).
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was supported by the Swedish National Space Board. The topical editor, Georgios Balasis, thanks Nick Sergis
and one anonymous referee for help in evaluating this paper.
References
Arthur, C., McPherron, R., and Means, J.: A comparative study of three
techniques for using the spectral matrix in wave analysis, Radio Sci., 11,
833–845, 1976.Boardsen, S. A., Anderson, B. J., Acuña, M. H., Slavin, J. A., Korth, H.,
and Solomon, S. C.: Narrow-band ultra-low-frequency wave observations by
MESSENGER during its January 2008 flyby through Mercury's
magnetosphere, Geophys. Res. Lett., 36, L01104, 10.1029/2008GL036034, 2009.Boardsen, S. A., Slavin, J. A., Anderson, B. J., Korth, H., Schriver, D., and
Solomon, S. C.: Survey of coherent ∼ 1 Hz waves in Mercury's inner
magnetosphere from MESSENGER observations, J. Geophys. Res.-Space, 117, A00M05, 10.1029/2012JA017822, 2012.
Dungey, J.: Electrodynamics of the outer atmospheres, Rep. 69, Ions, Res.
Lab.
Pa. State Univ., University Park, 1954.
Dungey, J. and Southwood, D.: Ultra low frequency waves in the magnetosphere,
Space Sci. Rev., 10, 672–688, 1970.Hughes, W. J.: Magnetospheric ULF Waves: A Tutorial with a Historical
Perspective, in: Solar Wind Sources of Magnetospheric Ultra-Low-Frequency
Waves, edited by: Engebretson, M. J., Takahashi, K., and Scholer, M.,
American Geophysical Union, Washington, D. C., 10.1029/GM081p0001,
1994.
James, M. K., Bunce, E. J., Yeoman, T. K., Imber, S. M., and Korth, H.: A
statistical survey of ultralow-frequency wave power and polarization in the
Hermean magnetosphere, J. Geophys. Res.-Space, 121,
8755–8772, 2016.
Liljeblad, E., Sundberg, T., Karlsson, T., and Kullen, A.: Statistical
investigation of Kelvin-Helmholtz waves at the magnetopause of Mercury,
J. Geophys. Res.-Space,, 119, 9670–9683, 2014.
Liljeblad, E., Karlsson, T., Raines, J. M., Slavin, J. A., Kullen, A.,
Sundberg, T., and Zurbuchen, T. H.: MESSENGER observations of the dayside
low-latitude boundary layer in Mercury's magnetosphere, J. Geophys. Res.-Space,
120, 8387–8400, 2015.
Liljeblad, E., Karlsson, T., Sundberg, T., and Kullen, A.: Observations of
magnetospheric ULF waves in connection with the Kelvin-Helmholtz instability
at Mercury, J. Geophys. Res.-Space, 121, 8576–8588,
2016.
Means, J. D.: Use of the three-dimensional covariance matrix in analyzing the
polarization properties of plane waves, J. Geophys. Res., 77,
5551–5559, 1972.Russell, C. T.: ULF waves in the Mercury magnetosphere, Geophys.
Res. Lett., 16, 1253–1256, 10.1029/GL016i011p01253, 1989.
Samson, J., Jacobs, J., and Rostoker, G.: Latitude-dependent characteristics
of
long-period geomagnetic micropulsations, J. Geophys. Res., 76,
3675–3683, 1971.
Samson, J., Harrold, B., Ruohoniemi, J., Greenwald, R., and Walker, A.: Field
line resonances associated with MHD waveguides in the magnetosphere,
Geophys. Res. Lett., 19, 441–444, 1992.Sibeck, D. G., Baumjohann, W., Elphic, R. C., Fairfield, D. H., Fennell, J.
F., Gail, W. B.,
Lanzerotti, L. J., Lopez, R. E., Luehr, H., Lui, A. T. Y., Maclennan, C. G., McEntire, R.
W., Potemra, T. A., Rosenberg, T. J., and Takahashi, K.: The magnetospheric
response to 8-minute period strong-amplitude upstream pressure variations,
J. Geophys. Res.-Space,, 94, 2505–2519, 1989.
Southwood, D.: Some features of field line resonances in the magnetosphere,
Planet. Space Sci., 22, 483–491, 1974.
Southwood, D., Dungey, J., and Etherington, R.: Bounce resonant interaction
between pulsations and trapped particles, Planet. Space Sci., 17,
349–361, 1969.