GLOBAL IMAGING OF PROTON AND ELECTRON AURORA IN THE FAR ULTRAVIOLET.

S. B. MENDE, H. U. FREY, T. J. IMMEL Space Sciences Laboratory, University of California Berkeley, Berkeley, CA 94720 J.-C. GERARD, B. HUBERT University of Liege, Liege, Belgium B-4000 S. A. FUSELIER, Lockheed-Martin Advanced Technology Center, Palo Alto, CA, 94304

Outline IMAGE FUV instrument provides for the first time a global view of electron and proton precipitation. Imaging proton auroras on the dayside permitted the discovery of proton auroral spots caused by direct entry of protons at the high latitude lobe merging site during (IMF Bz >0) [Frey et al., 2002, Fuselier et al., 2002] and of detached dayside proton arcs [Immel et al., 2002, Burch et al., 2002].

Outline cont. 2. The role of protons in substorms. Introduction to a statistical analysis of a limited set of substorms. Data reduction to perform a “superimposed epoch analysis” of the substorm set. The mean properties of proton precipitation in substorms regarding latitudinal and magnetic local time (zonal) expansion and drift.

Particle energy in the magnetosphere is carried mainly by ions (mostly protons). Most intense auroras are generated by electrons however they are energized “on the way down” Electron precipitated energy is not representative of the energy of the plasma sheet or the ring current trapped plasma. Nightside protons generally have energies of >25 keV and are less affected by the auroral electric fields. Protons have significant role in substorm models. The cross tail current is carried by ions. The pre onset distorted magnetic field at the equator is thought to randomize the proton pitch angle thereby causing proton precipitation observed prior to the onset of the substorm. Thus if we intend to explain substorms then we must understand the behavior of protons during substorms.

In spite of the many years of satellite based in situ exploration of protons, relatively very little is known about the dynamics of proton precipitation. Most proton measurements were made by ground based meridian scanning photometers measuring the H Balmer lines. From such measurements it is known that: 1. Pre-substorm protons are on the equatorward boundary of the oval. 2. Substorm onset and the break-up arc occur in the region of the proton precipitation. (Closed field lines) 3. During break up and after the proton aurora expands poleward. Note: that in the presence of bright break-up aurora the reliability of the visible light, ground based, proton auroral observations are always controversial.

From a single case study using the IMAGE data we have corroborated the ground based studies (Mende et al., 2001). In this report we are describing a study involving many substorms. Although the purpose of the study is to investigate proton auroras we will make comparisons to simultaneous electron auroras.

A superimposed epoch analysis of substorms observed in 2000 and early 2001 was performed by : 1. Re-plotting substorms in Magnetic-latitude/Magnetic- Local-Time coordinates. 2. The substorms were re-positioned in (MLT) so that the onset location was repositioned to MLT = 0. 3. A gaussian of the form: y = a0 e -((x - a1)/ a2)2 + a3 + a4 x + a5 x2 where x is the magnetic latitude and y is the intensity.

A set of 38 substorms were chosen for the statistical analysis. Criteria of choice: 1.Significant intensity Enhancement in WIC (electrons) at onset. 3. Discernable poleward propagation after onset. 2. No prior major substorm for at least 1 hour. The onset location (Mag. Lat. and MLT) of the electron precipitation was determined from WIC data.

2000/193 - 9:59:13

L A T MLT 2000/1939:59:13

Input data in Keogram form (left)and equivalent Gaussian fit (right) Input data in Keogram form (left)and equivalent Gaussian fit (right). Note good correspondence in general but we lose the “double auroras” on the right.

Example of output data: Amplitude (a0) of Gaussian for WIC taken at relative MLT=0 (I.e at the onset MLT). Note: Large WIC intensity increase at onset. The pre-event in WIC is unusual.

Example of output data: Amplitude (a0) of Gaussian for SI-12 taken at relative MLT=0 (I.e at the onset MLT). Note: Moderate enhancement SI-12. Strong pre-cursor activity with SI-12 (often observed)

The magnetic latitude of the peak of the Gaussian (a1) at the onset MLT (top). Poleward expansion is minimal and the SI-12 (protons) and WIC (electrons + protons) track each other well.

The magnetic latitude width of the peak of the Gaussian (a2) at the onset MLT (bottom). Large increase at onset with WIC wider than SI-12.

SI-12 (protons) drift primarily dawnward electrons expand in both directions

WIC auroral intensity plot (left) and ratio of intensity to pre-substorm value (right) for each substorm. The mean curve is the solid line. The average intensity enhancement is about a factor of 5.

SI-12 auroral intensity plot (left) and ratio of intensity to pre-substorm value (right) for each substorm. The mean curve is the solid line. Note: 1. The intensity enhancement is only of about a factor of two. This could be interpreted to signify that substantial proton precipitation is present prior to onset. 2. Pre-substorm average is < 1 erg but after onset several ergs are observed. (For an average nightside aurora 25 counts =1 erg cm-2 sec-1.)

Plot of Gaussian parameter a1 (latitude of the peak) left and a2(width of the peak) for the Substorm set. Blue =WIC, red=SI-12. Solid line WIC mean and broken line SI-12. Note: Latitude of protons is lower prior to onset. After onset they track. Width of electrons becomes narrow at onset and protons lead the widening for the first 15 minutes.

Statistics of the intensity just prior and at peak of intensity at Relative MLT = 0. Largest substorms occur at lower latitudes (WIC and SI-12). SI-12 has significant pre-substorm intensity which increases towards lower latitudes.

Ratio of peak to pre-existing aurora is relatively constant with latitude for SI-12. It increases with latitude for WIC.

Intensity pre onset and peak as a function of the actual Magnetic Local Time of the initiation point.

Ratio of intensity peak and pre-onset as a function of the actual Magnetic Local Time of the initiation point.

Examples of Relative MLT drift path for SI-12 And for WIC. Following an injection the auroras Expand in MLT longitude. More often than not they drift preferentially in one direction instead of just expanding. Electron auroras are caused by inverted V or Alfven Wave processes between the equator and the low altitude atmosphere. Protons are relatively immune from being perturbed by those fields and proton auroras may represent the Magnetospheric particle distributions and dynamics.

Distinct duskward trace in WIC (electrons).

Dawnward moving proton injection and dawn-dusk expansion in WIC

Complex example of multilple injections Complex example of multilple injections. Note pre-existing aurora at other MLT.

Strong pre-cursor proton precipitation and dawn-ward propagation.

Pre-cursor proton precipitation and dusk-ward propagation after injection.

Very intense proton injection

Summary of 38 cases examined : AVERAGE SI-12 MOTION IS 3.09 MLT/UT DAWNWARD AVERAGE WIC MOTION IS -0.37. (SLIGHTLY DUSKWARD) This is contrary to expectations based on gradient and curvature drift in the presence of a dawn/dusk electric field. What parameters does the direction of motion depend on?

SMALL SAMPLE DATA SET WAS PLOTTED AGAINST IMF. There is some trend of SI-12 with By but more data needs to be looked at to arrive at a definitive result.

No obvious trend with Bz but more data needs to be looked at.

Does the intensity of the substorm depend on the distortion of the pre-substorm magnetic field? If the pre-onset proton intensity were related to the distortion of the magnetic field then the substorm intensity would be proportional to it.

Summary. 1. Pre-substorm protons are on the equatorward boundary of the oval. 2. Substorm onset and the break-up arc occur in the region of the proton precipitation. (Closed field lines) 3. During break up and after the proton aurora expands Poleward. This expansion is very similar in both components. 4. Surprise. There is no evidence for saying that either electrons or protons lead the poleward expansion (Mende et al., 2001). 5. The average intensity enhancement in WIC is about a factor of 5. 6. The intensity enhancement of SI-12 is only of about a factor of two. This could be interpreted to signify that substantial proton precipitation is present prior to onset.

Summary continued. 8. Pre-substorm average intensity of protons is just under 1 erg but after onset they can be several ergs (sometimes ~5). This is much higher than the averages observed by in situ particle detectors. 9. Latitude width of electrons becomes narrow at onset and protons lead the widening for the first 15 minutes. 10. Largest substorms occur at lower latitudes (both WIC and SI-12). 11. SI-12 pre-substorm intensity also tends to be higher at low latitudes. 12. (Surprise) average SI-12 motioon is 3.09 MLT/UT DAWNWARD 13. Average WIC motion is insignificant. 14. There is some trend that SI-12 drift direction correlates with By but more data needs to be looked at to arrive at a definitive result.