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Physics of the Auroral Zone Electrons II (PHAZE II)

Launch date: 10 February, 1997

Launch time: 5:17 UT

Launch team: Principal Investigators: Arnoldy, Widholm (UNH); LaBelle, McAdams, Trimpi (Dartmouth); Kintner, Powell (Cornell)

Vehicle apogee: 945.3km approximately 556 seconds after launch

Location: Poker Flat, AK (65.13¡ N, 147.48¡ W)

Instrument list: Full Complement of Particle and Field Instruments: High Resolution Electron and Ion Detectors (UNH), VLF, ELF and DC Measurements (0-20 kHz) made with the Electric Field Experiment (Cornell), Waveform Snapshot Receiver, High Frequency Electric Field Instrument (Dartmouth)

Sample Data    Photos    Publications

Main Science

The Dartmouth high-frequency receiver on PHAZE II provided fully resolved waveform measurements of the component of the electric field parallel to the background magnetic field. This allowed auroral Langmuir waves and related phenomena to be fully resolved.

1. The nature of the waves depends on the ratio of the plasma frequency to the electron gyrofrequency.  At high altitudes where the plasma frequency is less than the gyrofrequency, called the underdense regime, the plasma waves occur just below the plasma frequency and are organized into bands: multiple narrowband structures which are constant in frequency and last from a fraction of a second to tens of seconds. Sometimes these bands are punctuated by an intense wave burst when their frequency matches the plasma frequency, and sometimes no such burst occurs.  We put forth that these bands represent conversion of auroral Langmuir waves to whistler mode in the inhomogeneous plasma. (The plasma frequency is the upper bound of the whistler mode in the underdense plasma regime, in which these modes are naturally connected.)  Sometimes the causative Langmuir waves are intercepted by the rocket and observed as a plasma frequency burst, and at other times the rocket misses the causative Langmuir wave burst. As is well known, auroral Langmuir waves are sporadic and bursty in nature, and hence the mode-converted whistler waves just below the plasma frequency in the underdense plasma consist of discrete bands rather than a continuous emission. Modeling using the WHAMP code supports this hypothesis.
   See McAdams et al., 1999, for details.
2. At low altitudes where the plasma frequency exceeds the gyrofrequency, called the overdense regime, the plasma waves occur just above the plasma frequency. They are also organized into narrow-band features, but these are not constant in frequency but rather decrease in frequency with time. They last typically for 100 ms or so. They come in pairs or sets of up to five or six parallel emissions evenly spaced in frequency, changing in frequency together. We call these wave structures "chirps."  We put forth that they arise from trapping of the nearly parallel (but slightly oblique) Langmuir waves in field aligned density cavities. The Langmuir wavevector is primarily parallel to the field and constant, so as to resonantly interact with the auroral electron beam, but there is a small component of the wavevector perpendicular to the magnetic field which can reflect from the sides of the density cavity and interfere with itself. The density cavity thereby impresses an eigenmode frequency structure on the trapped Langmuir waves. Some fraction of the wave energy escapes and carries this frequency structure with it as it propagates away, though its lifetime is short due to heavy damping once the wavevector refracts in the inhomogeneous plasma. Theoretical calculations support this hypothesis, showing that the observed frequency spacing of the wave modes 
   is predicted by the observed density cavity size and depth, electron beam velocity, and electron density. 
   For more details about the experimental data, see McAdams and LaBelle,1999. 
   For more details about the theory, see McAdams et al., 2000.
3. The high frequency experiment on PHAZE-II provided electron density measurements from observations of wave cutoffs. Such measurements during times of lower hybrid solitary structures confirmed that those structures are associated with density depletions. Using wave cutoffs to infer the density is not susceptible to effects of plasma inhomogeneity on collected currents, unlike previous measurements using Langmuir probes, which were controversial. 
   For details, see McAdams et al., 1998.
4. The high frequency receiver on PHAZE II also remotely sensed LF whistler mode emissions well below the local gyrofrequency or plasma frequency, apparently generated at greater altitudes in the aurora.  Surprisingly, these emissions are highly structured, consisting of multiple discrete features rising or falling in frequency, not unlike
   chorus observed at VLF associated with trapped electrons or fine structure of auroral kilometric radiation.  The frequency range, hundreds of kHz, is comparable to AKR but much greater than VLF chorus. In some cases, harmonic emissions were observed.
   For details, see LaBelle et al., 1999.
5. The high frequency receiver on PHAZE II detected numerous Langmuir wave bursts at the local plasma frequency. Within each burst the electric field amplitude is sampled numerous times. Statistics of the bursts reveal a Gaussian dependence of probability of occurrence (logarized) versus square of the electric field amplitude, but in more than half the cases, the distribution switches to an E^-2 power law at large amplitudes. A Gaussian dependence is predicted by stochastic growth theory. The power-law tails may indicate a nonlinear turbulent wave dynamics. These data have been presented by Samara and LaBelle, 2001, and a paper is in progress.

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72. McAdams, K.L., J. LaBelle, P.W. Schuck, and P.M. Kintner, PHAZE II observations of lower hybrid burst structures occuring on density gradients, Geophys. Res. Lett., 25, 3091, 1998.

77. McAdams, K.L., and J. LaBelle, Narrowband structure in HF waves above the Langmuir frequency in the auroral ionosphere, Geophys. Res. Lett., 26, 1825, 1999.

78. LaBelle, J., K.L. McAdams, and M.L. Trimpi, Structured low and medium frequency whistler mode emissions in the auroral ionosphere, J. Geophys. Res., 104, 28101, 1999.

79. McAdams, K.L., J. LaBelle, M.L. Trimpi, P.M. Kintner, and R.A. Arnoldy, Rocket observations of banded structure in HF waves near the Langmuir frequency in the auroral ionosphere, J. Geophys. Res., 104, 28109, 1999.

80. McAdams, K.L., R.E. Ergun, and J. LaBelle, HF Chirps: Eigenmode trapping in density depletions, Geophys. Res. Lett., 27, 321, 2000.