Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983-1984 and 1992-1993. This higher-frequency component is apparently triggered by solar wind transients with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2/3 power and in the other as time to the 4/5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of approx. 130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at approx. 70 AU when the transient shock collided with it, the true distance to the source region was probably closer to approx. 115 AU

Belcher, J. W.

Goodrich, C. C.

Kulkarni, R.

Lazarus, A. J.

Lyon, J.

McNutt, R. L., Jr.

English

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NASA-CR-204910
(_ Pergamon
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0273-1177(95)00352-5
Adv. Space Res. Vol. 16, No. 9, pp. (9)303-(9)306, 1905
Copyright _ 1995 COSPAR
Printed in Great Britain. All rights reserved.
0273-1177/9559.50 + 0.00
THE DISTANCE TO THE HELIOSPHERIC
VLF EMISSION REGION
R. L. McNutt, Jr,* A. J. Lazarus,** J. W. Belcher,** J. Lyon,***
C. C. Goodrich? and R. Kulkarnit
* The Johns Hopkins University Applied Physics Laboratory, Laurel,
MD 20723, U.S.A.
** MIT Department of Physics and Center for Space Research, Cambridge,
MA 02139, U.S.A.
*** Department of Physics and Astronomy, Dartmouth College Hanover,
NH 03755, U.S.A.
t Department of Astronomy, University of Maryland, College Park,
MD 20742, U.S.A.
ABSTRACT
Two mejor episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager
spacecraft in 1983184 I11 and 1992/3/2/. This higher-frequency component is apparently triggered by
solar wind transients/3,4/with sufficiently large spatial extents and energies to continue to propagate as
shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in
the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather,
it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution
the shock moves as time to the 2/3 power 151 and in the other as time to the 4/5 power. Using these
models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the
1992 emission is consistent with an emission region distance of-130 AU (assuming no additional
slowing of the shock in the heliosheath). If the termination shock was at -70 AU when the transient
shock collided with it, the true distance to the source region was probably closer to -115 AU.
INTRODUCTION AND CURRENT STATUS OF EMISSIONS
In 1984 low frequency (-3 kHz) radio emissions in the outer heliosphere were detected by the Plasma
Wave experiment (PWS) on both Voyager spacecraft. The strong emissions were "on" for -200 days in
late 1983 and early 1984 and interpreted as coming from the vicinity of the termination shock of the solar
wind IlL Gurnett et al. /2/ reported a new high-intensity event observed in mid-1992 through early 1993.
Detected by both Voyager spacecraft, these emissions are similar in spectral and temporal structure to
those detected in 1983/4 but are the most intense emissions detected to date. McNutt hypothesized that
the high speed streams observed in the solar wind with the Plasma Science (PLS) experiment on Voyager
2 in 1982/3 were responsible for triggering the 1983/4 emission and derived bounds of -70 AU to -140
AU to the emission region/3,4/. This "trigger hypothesis" was predictive: "Given the passage of such a
stream by Voyager 2, we suggest that a strong component of radio noise will again be registered by the
Plasma Wave experiments on the two Voyagers in less than a year from the passage."/3/
(9)303
https://ntrs.nasa.gov/search.jsp?R=19970022895 2020-06-16T02:17:35+00:00Z
(9)304 R.L. McNutt,Jretal.
1991 SOLAR ACTIVITY AND THE 1992/3EMISSIONS
A possibility for testing the trigger hypothesis occurred on 26 May 1991. At 35 AU, the Voyager 2 PLS
experiment detected an anomalously strong shock, apparently associated with the increased level of solar
activity in March 1991. McNutt et al. /6/ predicted a new strong episode of radio emission commencing
near the end of 1991. By early 1992, there had been no new onset; however, a later shock did result in a
renewal of the emissions/2/. By applying the trigger hypothesis, Gurnett et al. /2/ identified the shock as
the the one responsible for the large Forbush decrease that passed the Earth on day 147 of 1991 and
Voyager 2 about day 250/7/and derived a distance to the heliopause of 116 to 177 AU.
THE TRIGGER EQUATION AND THE IDENTIFICATION OF THE TRIGGER
For a disturbance propagating radially from the sun at a speed Vd_t, the time delay At between the time of
observation at the spacecraft (located at a heliocentric distance of robs) and the time of the observation of
the the VLF radio emission can be easily calculated. Neglecting small corrections due to light transit time
and spacecraft motion
r,owrcr
fAt =-tubs -- ttrisse r _ (1)
Vdist
rob_
Uncertainty in a good quantitative estimate of the heliocentric distance to the emission site rsource is
driven by lack of knowledge of vdist as a function of r. Estimates of rso, rce to date/2,3,4/have been
based upon constant values of Vdist.
Unique association of events on the sun with shock waves seen by interplanetary spacecraft has remained
problematic. Gurnett et al. have argued that the triggers are associated with the Forbush decreases seen at
Earth orbit for both the 1983/4 and 1992/3 VLF events/2/. Solar wind data from the PLS experiment on
Voyager 2 shows the shock associated with the cosmic ray flux decrease on day 251 of 1991 traveling at
548 5:10 km s-l; the spacecraft heliocentric distance was 35.4 AU. The major Forbush decrease of 1982
on day 195 suggested for the 1982/3 trigger/2/passed Voyager 2 some 6 months prior to the trigger
suggested originally/3,4/. However, the fast streams identified by McNutt were also responsible for
keeping the cosmic ray flux depressed/8/and are associated with a second large increase in solar activity
during that year. Dynamical evolution of several decrease-producing shocks driven by the high speed
streams/3,4/is likely to have produced the "correct" major high-speed trigger structure/9/.
BLAST WAVES AND SIMILARITY SOLUTIONS
Propagation of anomalously strong transient shocks in the interplanetary medium can be approximated by
spherically symmetric blast waves. So-called similarity solutions exist if only two dimensional quantities
(as well as other dimensionless parameters) are required to specify the problem/10/. Hydrodynamic
quantities behind the shock are then only functions of a single similarity parameter I? -= rt-a. For a
medium with density varying as a power of r, these solutions do not exist for a constant speed outflow
such as in the solar wind case. However, suppose we make a "local" Galilean transformation to the rest
frame of the solar wind. Let primed quantities refer to the solar wind frame (taken to have a constant
radial velocity v). The undisturbed density varies as p'(r') = por_/(r" + vt) 2, and the blast wave occurring
at r = 0 and t = t" = 0 convects outward with the solar wind. The shock front moves out more rapidly
than the undisturbed solar wind and is at a location rshock" = rssoc k - vt. Hence, there exist two limits to
the undisturbed density in the primed frame of reference:
2 (II) r' 4(I) r' >> vt p'(r') = r ''2 --> a = _ _ vt p'(r') = t-2 ---> ot = _ (2)
The Distance to the Heliospheric VLF Emission Region (9)305
These two cases correspond to similarity solutions in the solar wind frame (specified by the blast wave
energy E = const, P0, and the specific heat ratio y): the first to a shock speed so much in excess of the
solar wind speed that the latter is irrelevant/11/and the latter to a density that drops everywhere in time
because we are viewing the surrounding medium from the moving frame. The true solution should have
a = a(r, t) varying from 2/3 to 4/5 in the vicinity of the shock front of the blast wave. Let r = 0, t = 0
correspond to the initial blast wave (day 145 of 1991) and observation of the shock by Voyager 2 to
r = 35.4 AU at t = 0.2904 yr (day 251 of 1991). Solving for the undisturbed solar wind speed we obtain
487 and 427 km s-I for a = 2/3 and 4/5, respectively (a = 1 corresponds to constant propagation at 548
km s-l and overconstrains the problem, i.e. a constant value of Va_st is not consistent with the
observations). The corresponding locations of the shock on day 188 of 1992 (t = 1.1178 yr) when the
radio noise turned on are 128.5 and 127.8 AU, respectively. Simple propagation of the shock at a
constant speed from the Voyager 2 observation gives a location of 131.0 AU.
TRANSIENT PROPAGATION IN THE HELIOSHEATH
If the region of the heliopause is the source of the emissions/2/, then two complicating factors arise: (1)
there is a change in propagation speed of the triggering disturbance at the termination shock and (2) the
location of this change (the termination shock) is not known. Using the Rankine-Hugoniot conditions and
ratio of specific heats y = 5/3, the transmitted disturbance speed is cs + _'sheath= 0. 809 Vsola r wind for
weak disturbances. As the strength of the transient shock is increased, the transmitted transient speed
increases toward that of the ambient unshocked solar wind speed /12/. Then for nearly constant
propagation speed through the heliosheath, the true source distance is
rsource = O. 809rsource'+ O. 191rshoc k. Here rsource' is the source distance inferred from timing with no
correction and rshock is the distance to the termination shock. For an inferred source distance of -128 AU
and a shock distance of 62 AU/13/, the inferred true distance to the source is -115 AU. Larger distances
to the shock give a limiting distance of 128 AU (the inferred distance), close to the value expected for
static pressure balance by the ram pressure alone of the interstellar medium/3,4/. For these cases, the
Voyager 1 and 2 spacecraft may reach interstellar space during their operational lifetimes.
NUMERICAL MODEL
We have applied a full time-dependent MHD simulation code to the problem of transient propagation
through the heliosphere. The simulation grid covers the region from 300 AU upwind to 600 AU
downwind; the inner boundary is taken at 40 AU. We assume a constant speed (400 km s-l) solar wind
with a density of 5 cm -3 at 1 AU. For interstellar conditions, we take a density of 0.05 cm -3, flow speed
of 26 km s-1 and a temperature of 104K. An interstellar magnetic field with equal components of 1 _uG
along each axis is assumed. The code was run to 200 years at which time the upwind region is close to a
steady state. The simulation gives a relatively large heliosphere with the termination shock near -120 AU
and heliopause along the stagnation line near-160 AU.
We inject a solar wind transient and follow its evolution. We assume the transient propagates along the
stagnation streamline and fills a conical region with a half-angle of 45 °. To characterize the transient, we
increase the density by a factor of 2 and step the velocity to 700 km s-1 for 7 × I05 s. The velocity and
density are then linearly ramped down over 10s s. Figure 1 shows a contour plot of the time evolution of
the density. Features moving outward at a constant speed give rise to features that slope linearly upward
to the right in this kind of plot. The vertical clustering of contour lines on the right hand side of the plot
indicates the heliopause; a similar clustering somewhat to the left is the termination shock, where the
density increases by a factor of 4. The transient disturbance can be seen as the S-shaped jogs in the
contour lines that propagate outward to the termination shock. Preliminary analysis suggests good
agreement with a -- 2/3 in the solar wind frame. Further work is required to produce a simulation
quantitatively consistent with solar wind /14/and VLF /1,2/ observations and known properties of the
Very Local Interstellar Medium (VL?SM)/15/.
(9)306 R.L. McNuu, Jretal.
...... .Lo.qor,D.en.,;ty:_,09,_i,.d.O;_ectlon......
J
.... • • •l]ull . i# ._i . i , . , , l
5O 100 1_ 2O0 25O
R (At/)
3OO
Fig. 1. Density contours (as a function of space and time) for a solar wind transient
propagating in the global simulation (x, horizontal, is the distance from the sun in the upwind
direction for the interstellar medium; t, vertical, is the time in years from the injection at the
inner boundary at 40 ALD.
ACKNOWLEDGEMENTS
This work has been supported by NASA at JHU/APL under subcontract SC-A-38368 from MIT, under
Task I of Navy Contract N00039-91-C-001, and at MIT under NASA contract 959203 from JPL to MIT,
and under NASA grant NAGW- 1550.
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11. E. N. Parker, Interplanetary Dynamical Processes, John Wiley, New York, 1963.
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The Distance to the Heliospheric VLF Emission Region

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