Three Second Waves Observed Upstream Of The Earth´s
Bow Shock
X. Blanco-Cano1, C. T. Russell2, J. Ramírez1, and G. Le3
1
Instituto de Geofísica, UNAM, México D.F.; 2IGPP, UCLA, Los Angeles; 3Godard Space
Flight Center, Maryland.
Abstract. A new class of ULF waves were discovered in the foreshock from the ISEE magnetometer data by Le
et al. [1]. These unusual type of waves differ greatly from the more commonly observed 30 s waves, shocklets and
SLAMS. The new waves have periods near 3 s and always show a very narrow spectrum that is in contrast with the
lower frequency waves observed in the foreshock, which usually have broader spectra. Three second waves have
large amplitudes and are observed in the upstream region only when the interplanetary magnetic field intersects the
bow shock and when the plasma beta is high. Three second waves can be divided in three types. Isolated waves are
associated with reflected cold beams and are observed in regions where the magnetic field is very quiet. Superposed
waves are associated with more intermediate ion distributions, and are superposed on non steepened lower
frequency waves. Irregular waves are associated with diffuse ions and are observed as trains surrounded by an
irregular magnetic field. Three second waves are generated by the right-hand non resonant instability, being righthanded and propagating downstream in the plasma frame. This instability grows due to the interaction of the solar
wind distribution with a reflected ion beam. In this work we map the location of the different types of three second
waves with respect to the bow shock, and study their evolution in the foreshock. We also study which instability is
generating the lower frequency waves observed with the superposed 3 s waves.
INTRODUCTION
WAVE CHARACTERISTICS
This paper continues the study of three second
waves discovered in the foreshock from the ISEE
magnetometer data by Le et al. [1]. These unusual
type of waves differ greatly from other ultra low
frequency (ULF) waves, such as 30 second waves,
shocklets, and SLAMS [2, 3]. They have periods ~3 s
and always show a very narrow spectrum that is in
contrast with the broader spectrum of other ULF
waves. They are always right-hand nearly circularly
polarized in the spacecraft frame and are convected
downstream by the solar wind. Three second waves
are observed in the upstream region only when the
interplanetary magnetic field intersects the bow shock
and when the plasma beta is high. ULF waves are
generated in the foreshock by kinetic ion instabilities
produced by the interaction of the solar wind with
suprathermal ion beams. Their study is important
because they play an active role in the interaction of
the solar wind with the bow shock; they participate in
wave-particle interactions, and in the transmission of
wave energy from one region to another.
Three second waves can be found in three different
environments [4]. Figure 1 shows an example of each
type of wave, and of the suprathernal ions (bottom
panels) that are observed with the waves. Data are
from ISEE magnetometer [5] and ISEE fast plasma
experiment [6]. Isolated waves are observed in regions
where the magnetic field is very quiet (Fig. 1a).
Superposed waves (Fig. 1b) are observed on top of
lower frequency (10-2 Hz) quasi-sinusoidal
nonsteepened waves. Irregular waves (Fig. 1c) are
observed as trains surrounded by a perturbed magnetic
field. Three second waves are associated with different
types of suprathermal ions. While isolated waves are
associated with reflected field-aligned cold fast beams,
irregular waves are accompanied by more isotropic
diffuse hot ions, with slow drift speed. Superposed
waves are accompanied by ions whose characteristics
are intermediate. Three second waves propagate at
small angles ≤30° with respect to the background
magnetic field and have amplitudes δB≈ 2-7 nT.
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
501
(a)
0841:51.90
2343:41.83
0842:03.90
X
800
1600
2400
(c)
(b)
800
1600
2400
UT
2343:53.82
0534:28.95
2400
Y
SUN
0534:01.76
800
1600
2400
800
1600
2400
1600
800
800
1600
2400
FIGURE 1. Three second waves observed by ISEE 1: (a) isolated waves (0841:56-0842:44, and 0844:30-0846:00 on
November 3, 1978), (b) superposed waves (2343:33-2343:56, 2344:09-2344:4, and 2345:52-2346:49 on September 7, 1979),
and (c) irregular trains (0533:45-0534:44, and 0536:45-0537:45 on October 2, 1978). Bottom panels show suprathermal ions
associated with the waves. Distributions are given as contours of constant phase-space density separated logarithmically (two
contours per decade). The solar wind appears as tighten contours at positive vx. Concentric circles indicate speeds of 800,
1600 and 2400 km/s. The arrow through the solar wind distribution is the projection of the average magnetic field.
SPATIAL VARIATION OF WAVES
are observed immediately upstream from the bow
shock, as well as near ISEE orbit apogee, far from the
nominal bow shock position [1]. In order to investigate
if isolated, superposed and irregular waves permeate
the same regions or not, we find the position of the
waves with respect to the bow shock. The position of
the shock was found using the model of Farris and
Russell [7]. Figure 2 shows dx, the wave position with
respect to the bow shock nose, and δB/Bo the
normalized amplitude of three second waves observed
by ISEE 1. dx is the distance along the X direction,
and as in all cases the solar wind velocity was mainly
along X, the distance from the shock along the solar
wind flow direction can be approximated as dx. It is
possible to see that superposed and irregular waves are
found at distances ≤ 3 RE (RE ≡ earth radius). In
contrast, most isolated waves are found at much larger
distances, reaching ≈7 RE. The amplitudes of
superposed and isolated waves are in between 0.1-0.8
Bo, while irregular waves have amplitudes ≈0.5 up to
1.2 Bo.
The properties of waves and plasma are position
depending within the foreshock. Three second waves
The fact that different types of three second waves
are found at different distances from the shock is in
We compared three second wave properties
(frequency, wavenumber, polarization, magnetic
compression, and noncoplanar ratio) with the
characteristics of kinetic ion instabilities and showed
that three second waves can be identified as the righthand non resonant instability, being right-handed and
propagating downstream in the plasma frame [4]. We
found that isolated and superposed waves can be
generated locally by the right-hand nonresonant
instability that grows due to the solar wind interaction
with the reflected beams associated with these waves.
In contrast, the diffuse ions observed with irregular
waves can not generate the right-hand nonresonant
mode, and we suggested that irregular waves are
generated upstream of where they are observed by the
right-hand nonresonant instability.
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wave growing which can take place as the waves
convect with the solar wind.
Comparison of observed three second wave
properties with ion kinetic instabilities allowed us [4]
to identify the right-hand non resonant instability as
the mode that is generating three second waves. In our
previous work we concluded that while isolated and
superposed waves can be generated locally by their
associated suprathermal ions, irregular waves must be
generated upstream from where they are observed. The
fact that irregular waves are observed closer to the
shock and have larger amplitudes supports our
prediction. The growth of waves in an unstable region
is not instantaneous, and in fact, waves can grow as
they convect towards the bow shock, providing that
the free energy is available from the suprathermal ions.
Thus, irregular waves are cases of “well-developed”
isolated waves that were generated upstream and grow
as they approached the shock. In the parcels of plasma
where these waves are observed other perturbations
grow while approaching the shock creating the very
perturbed environment that is observed beside
irregular three second waves. This scenario is sketched
in figure 3.
dx(RE)
FIGURE 2. Wave amplitude (δB/Bo) and distance (dx)
from the bow shock along the X-direction (≈solar wind flow
direction). Solid dots correspond to isolated waves, triangles
to superposed and diamonds to irregular waves.
agreement with the association of three second waves
with suprathermal ions, and with the magnetic
environment surrounding the three types of waves.
Isolated waves are observed with reflected cold beams
and are found in regions where the magnetic field is
very quiet. Reflected cold beams are produced in the
bow shock and can reach large distances upstream.
The fastest beams can reach the upstream edge of the
foreshock. Far from the bow shock there are few
waves and this is reflected in the cold non-disturbed
ion distributions observed with isolated waves, which
have not been heated by wave scattering. The
interaction of these beams with the solar wind makes
the plasma unstable and waves start to grow. These
waves are convected by the solar wind downstream,
where via wave-particle interactions they start to heat
the ion distributions. Closer to the shock, wave activity
increases and wave-particle interactions result in ion
scattering leading to the wider distributions associated
with superposed three second waves. As we discuss
below, in these regions the plasma can be unstable to
more than one mode leading to the generation of the
lower frequency waves observed with superposed
three second waves. Even closer to the shock (dx ≤
2 RE ), where irregular waves are observed, ion
distributions are hotter and isotropic as a result of
continuos ion scattering due to the waves that
permeate the deep foreshock. The fact that irregular
waves have larger amplitudes than superposed and
isolated waves is consistent with previous findings [8]
that show that closer to the shock ULF waves have
larger amplitudes. This can be explained as a result of
Bsw
RH nonresonant and
RH resonant
Superposed
cold
beam
hot diffuse
ions
solar wind
distribution
Isolated
RH nonresonant
instability
Irregular
Vsw
Generated
upstream
FIGURE 3. Drawing sketch of position, generation, and
evolution of three second waves.
LOWER FREQUENCY WAVES
The ULF waves that are observed near three
second superposed waves have frequencies f∼0.020.03 Hz. In contrast to three second waves, they are
left-handed and elliptically polarized in the spacecraft
frame. As stated above, in regions with superposed
waves the suprathermal ions have intermediate
503
The lower frequency (f∼0.02-0.03 Hz) waves
observed near superposed three second waves are
identified as the right-hand resonant mode.
1.0
γ/Ωp
0.8
0.6
RH non resonant
RH resonant
0.4
0.2
0.00
0.05
nb/nsw
0.10
To our knowledge three second waves have only
been studied using ISEE data. It would be very
interesting to study in more detail the properties of
these waves with present missions such as CLUSTER.
0.15
FIGURE 4. Growth rate of the right-hand resonant and non
resonant instabilities as a function of beam density.
characteristics, i.e. they are field-aligned and have
suffered some heating by wave scattering.
ACKNOWLEDGMENTS
The interaction of the solar wind with these beams
can generate the right-hand resonant and the righthand non resonant instabilities. Figure 4 shows growth
rate as a function of beam density for the right-hand
resonant and non resonant instabilities. The beam
characteristics resemble the suprathermal ions
observed with superposed waves (Tb=30Tsw, vob=20VA,
more details are given in [4]). It is possible to see that
when the beam is sufficiently dense the right hand
nonresonant mode dominates, and when the beam
density is small (<0.04) the right-hand resonant mode
has the largest growth. Therefore, we believe that the
lower frequency waves observed with superposed
three second waves can be identified as the right-hand
resonant mode, which suffers a reversal in polarization
and is left-handed in the spacecraft frame. In regions
with superposed waves, beams with different densities
interact with the solar wind, leading to regions where
the nonresonant mode is dominant and regions where
the resonant mode has the higher growth.
We thank J. T. Gosling of LANL for kindly providing
the suprathermal ion data. We are grateful to J.
Newbury for helping us to process suprathermal ion
distributions.
REFERENCES
1. Le, G., C. T. Russell, M. F. Thomsen, and J. T. Gosling,
Observation of a new class of upstream waves with
periods near 3 sec, J. Geophys Res., 97, 2917-2925,
1992.
2. Greenstadt, E. W., G. Le, and R. J. Strangeway, ULF
waves in the foreshock, Adv. Space Res., 15 (8/9) 71-84,
1995.
3. Burgess, D., What do we really know about upstream
waves?, Adv. Space Res., 20 (4/4), 673-682, 1997.
4. Blanco-Cano, X., G. Le, and C. T. Russell, Identification
of foreshock waves with 3-s periods, J. Geophys. Res.,
104, 4643-4656, 1999.
CONCLUSIONS:
5. Russell, C. T., The ISEE 1 and 2 fluxgate
magnetometers, IEEE Trans. Geosci. Electron., GE-16,
239-242, 1978.
Three second waves can be divided in three types:
isolated, superposed and irregular. The waves are
associated with different suprathermal ions: Isolated
waves are accompanied by reflected ion beams,
superposed are observed with
intermediate
distributions, and irregular waves are associated with
diffuse ions.
6. Bame, S. J., J. R. Asbridge, H. E. Felthauser, J. P. Glore,
G. Paschmann, P. Hemmerich, K. Lehmann, and H.
Rosenbauer, ISEE-1 and ISEE-2 fast plasma experiment,
IEEE Trans. Geosci. Electron., GE-16, 216, 1978.
7. Farris M. H., and C. T. Russell, Determining the
standoff distance of the bowshock: Mach number
dependence, J. Geophys. Res. 99, 17681, 1994.
We found that irregular waves are observed closer
to the shock than isolated waves. This supports an
scenario that we proposed in the past [4] in which
isolated and superposed waves are generated locally
by the right-hand nonresonant instability, while
irregular waves are generated upstream of where they
are observed by the same instability.
8. Le, G, and C. T. Russell, The morphology of ULF waves
in the Earth´s Foreshock, in Solar wind sources of
magnetospheric Ultra Low Frequency waves, Ed. M. J.
Engebretson, K. Takahashi, and M. Scholer, Geophys.
Monograph 81, 81-98, AGU, 1994.
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